And by "best" I mean, "best at killing players."
I've read more than a few times that the Holmes basic edition is the most dangerous, mortality producing version of the game ever written. This is due in part to certain "expert level" monsters (like the purple worm, manticore, hydra, and vampire) being included in a game where the characters only go from levels 1-3.
Well, I beg to differ...I mean, dead is dead, right? If you're digested by a purple worm you're not anymore dead than being smacked for max damage by an ogre; there's a point of diminishing returns on that kind of thing (i.e. "death").
[unless, of course, you're killed by a vampire...then I suppose you can be "even more dead."]
But Moldvay's red book has plenty of monsters that will wipe out characters in the 1-3 range...many of whom are listed as standard wandering monsters for those very same levels. The idea that Moldvay's version of Basic D&D is somehow safer or "more balanced" is completely ridiculous. At least in Holmes characters with a high Dexterity will (on average) gain initiative (and thus, the upper hand or opportunity to run) in any encounter. In B/X, initiative is always a crap shoot.
The following list is my Top Ten Most Deadly Monsters from Moldvay's red book. Most of them are also my favorite monsters in the Moldvay set (these I've marked with an "*"). Not surprisingly, they are very real PC killers...only a true asshole of a DM would use these in an adventure for characters under 4th level, at least in the numbers listed in the rules (a singular, lone creature is much easier to deal with than a group, at least for a large adventuring party).
Oh, by the way...dragons (of any color) are NOT on the list. While I will be the first to say there aren't ENOUGH dragons in your average D&D campaign, dragons have such wildly varying ability that one can't really say whether they are consistently deadly (a stupid, sleeping, dragon of young age and no spell-casting ability isn't much of a threat if the party can get the drop on it and reduce its hit points before its first breath attack).
Here's the consistent badasses:
#10 Zombies: If there was any question in my mind whether or not "the damned dead" should be here, it was answered by last Thursday's decimation of adventurers. Unlike every other edition of D&D (including AD&D and Holmes), Moldvay's zombies are CHAOTIC (all undead in B/X are Chaotic), which is to say "unholy" and "evil;" probably the reason holy water is so effective on 'em. I already wrote how nasty these guys are...they beat out other 2HD monsters (like gnolls) due to their fearlessness (no morale checks) and immunity to sleep spells. Used in large numbers they are likely to take apart any 2nd level parties they encounter.
#9 Shadows: Again, a change-up from other editions of the game, B/X shadows are NOT undead, and thus NOT subject to turning...however, they are still immune to charm and sleepspells and being incorporeal, can only be hit by magic weapons. They show up on the 3rd level of a dungeon (1-8 appearing!); how many of your 3rd level character are carrying magic weapons? Strength drain is delicious and even if a party survives the confrontation, will probably be left deep in the dungeon in a weakened condition.
#8 Minotaurs*: I've always loved the minotaur as a monster; dug it in the legend of Theseus, dug it in Saturday morning cartoons (an episode of the old Godzilla, if I'm not mistaken), and loved Willingham's illustration in B2: The Keep on the Borderlands. A 6HD monster that gets a +2 on damage when using a weapon. Being larger than an ogre, it is immune to both charm person, hold person, and sleep, and will probably kill at least one or two PCs before being brought down, even by large parties. Minotaurs are also intelligent, and unlike other monsters "will pursue as long as its prey is in sight" (this one isn't distracted by dropped rations). Vicious...did I mention that the normal number appearing is 1-6? What the hell is this doing in the Basic game?
#7 Harpies*: As with minotaurs, I've always loved the harpy; I've been a fan of Peter S. Beagle's The Last Unicorn (both the book and the very faithful film adaptation) for years, and the harpy is an awesome villain...but which is the adventure module where, if PCs fail their save versus the harpy's song it comes and (automatically) "rips their eyes out?" One of the Slavers series maybe? Or the Master of the Desert Nomads? Regardless, that's the kind of encounter I love to see in adventure modules (and that some players...hi, Luke!...absolutely loathe). But if any monster should be a malicious, de-protagonizing bitch, it should be the harpy. Monsters that fly mean monsters that are hard to bring down (and that circle to keep out of range of spells). Three attacks per round (claw-claw-weapon) makes them exceptionally nasty, even without the charming. I used surgically-modified harpies in my Paschendale Necropolis adventure (no singing and no weapon attacks) and they still killed both hirelings and wounded several party members in nothing flat. The fact that they don't rate higher on this list should tell you something about numbers 1 though 6.
#6 Bears*: My love for the bear as a B/X monster is, I think, fairly well known. The only thing that doesn't rate them higher is their low "number appearing" stat (usually only 1, unless in their lair). Often totally underestimated...what? It's just a bear, right?...they will kill party members very quickly before they even know what hit 'em. Except for the black bear, all bears are larger than the ogre and are thus immune to the sleep/charm/hold spells of low level characters, and being animals are fairly immune to reasoning or negotiation (and since you usually only encounter ONE, they're generally NOT subject to morale checks!). Of the bunch, my hands down favorite is the polar bear (even the non-armored variety), because they seem so mundane...right up until they kill you. "Oh...and the bear hits you several times and does [*roll*roll*] ...30 points of damage to you! Holy crap!"
#5 Ghouls*: While these are a personal favorite (nothing says "terror" in the dungeon like a pack of flesh-eating undead) I almost never use them except in high level games or very small moderation. Why? Because they are Total Party Kills waiting to happen. 2HD creatures with claw-claw-bite ability are nasty enough...I've seen a half-dozen troglodytes with the same D4/D4/D4 take down two plate armored fighters and a plate-and-shield cleric without batting an eye. Ghouls do D3/D3/D3 with the same chance to hit, and every attack that hits forces a save versus paralysis (requiring a 12-14 save roll on the D20 for characters under 4th level). Did I mention they travel in packs of 1-6? And being undead they're immune to sleep/charm/hold? That gives 'em a leg up over the tentacled carrion crawler. Did I also mention that per Moldvay they start showing up on level two of the dungeon? Do you know what a 2nd level cleric needs to roll to turn a ghoul at 2nd level? A nine. Fairly long odds...and if you happen upon a lair (treasure type B = 2,000gp average), you'll encounter 2-16. That's a lot of diseased nails raking the flesh from your bones. 'Course it could be worse: in OD&D and AD&D being killed by a ghoul turns you into a ghoul!
#4 Mediums: 1st level magic-users come in packs of 1-4. The only reason they don't rate higher is it's always possible the PCs might get the drop on 'em and take 'em down with a sleep spell of their own. Otherwise, it should be short work for one of the mediums to get off a sleep spell and drop an entire adventuring party. Heck, a magic-missile might well finish off that rival party mage hiding in the back ranks, and if accompanied by their "master" (only a 3rd level magic-user in the B/X monster description!) the party may well find themselves trying to push their way through a web spell to get to said magic-users. In the lair (a school?) mediums are encountered in groups of 1-12...that's a lot of charm spells. Personally, I'm surprised it only rates as a 3rd level encounter.
#3 Lycanthrope: Werewolves*: Although these don't show up in B/X until the 4th level of the dungeon, they are present in the Basic book, and are one of my all-time favorite monsters. I almost never use them. Generally found singularly in old TSR adventure modules (a la the standard horror cinema "wolfman"), when used as written, they can be one hell of an encounter: number appearing 1-6 (2-12 in lair/wilderness). In addition, lycanthropes can each summon 1-2 normal animals to aid them and werewolves "summon normal animals to form large packs with them." On average that's nine monsters (3-4 werewolves and 5-6 normal wolves) or double that (around 18!) in the wilderness or dungeon lair. Any group of five or more has a 5HD, 30 hit point leader that does +2 damage (and is, of course, immune to sleep and charm and hold person spells...at least in wolf form), and all werewolves require silver or magic weapons to injure. Assuming you can tell which wolves are the lycanthropes and which are the normal wolves (how many silver arrows are the low level archers packing?). Wolf packs tend to maul the hell out of characters anyway (I saw three or four normal wolves take down a charmed ogre during a run of M1: Blizzard Pass) and werewolves fight and attack like dire wolves. Such an encounter with "average" numbers will kill several PCs, especially the lighter armored party members. And even should they run, wolves are some of the fastest pursuit critters in the game (180' move compared to the un-armored PC's 120' move). It would be a small matter for such scent hounds to run the PCs down.
#2 Owl Bears*: Probably my all-time, hands down favorite monster of the Moldvay Basic book, they are also probably the baddest of badasses. Cross a grizzly with a griffin and what do you get? A creature that can't be stopped by the spells available to characters level 1st through 4th and that can do up to 40 points of damage in a single round. Claw-claw-bite at D8/D8/D8 plus "bear hug" for 2D8...and did I mention they hunt in packs of 1-4? A normal grizzly is only ever found solo in a dungeon...you can encounter up to 4 times that many owl bears on the 4th level of a dungeon, and they will rip you to shreds. Bears of a feather flock together, I guess. Need it be mentioned that with 5 hit dice they're immune to charm, sleep, hold person, etc.? Oh, yeah...I already said that. When these bad boys come out, even 4th and 5th level fighters tremble in their boots.
#1 Medusa*: Another monster I almost never use. Interesting that the OD&D version had the lower body of a snake, like the classic gorgon of Greek myth...not sure why they changed it in later editions except possibly to not confuse it with the (confusingly-named) bull-like creature. Moldvay's description of the monster constantly refers to it in the singular, which I find strange as the number appearing is 1-3 (1-4 in lair). An average of 2 medusa per encounter, each one of which is 4HD with an auto-death attack (poison) AND and an auto-petrifaction effect (no attack roll necessary). The medusa (in numbers of 1-3) first show up on the 3rd level of the dungeon. What party of 3rd level characters is going to survive a wandering encounter with three medusa? That's just a ridiculously tough encounter...you might as well call 'em half-hit dice mind flayers. I feel mean just putting ONE medusa in an adventure; as I said, most of the time I just leave 'em completely out of the game. Too bad, though, because Perseus and the gorgon is probably my 2nd favorite Greek myth, right after Theseus and the minotaur.
All right, that's the list...and glad I am to get it off my chest. One of the monsters on this list will be featured in blog posts all week long, starting tomorrow, but for right now I'll let you contemplate the sadism of Tom Moldvay's "Basic" set and the death and destruction it is possible to unleash even before opening the "Expert" box. I know I did, back in the day, as I owned the Basic set probably for a whole year prior to getting the Cook/Marsh Expert rules.