All right, so I'm a bit late to the party on this one, but in joining with the semi-recent spate of bloggers around the Web listing their top 10 favorite D&D monsters, I thought I'd write up mine. Note that this list pertains to monsters published for 1e AD&D.
9. Ogre. As a player, I always considered ogres a "coming of age" challenge for PCs: These brutes were killers of low-level characters due to their high strength and damage potential, but once able to defeat an ogre your character had accomplished a great feat and had graduated from mere dungeon-fodder. Ogres are also every DMs' friend due to their inherent flexibility, as they are able to serve as lone cave-dwellers, or as hired muscle and encountered with groups of orcs and the like. My current DM uses breast-plated, warhammer-wielding ogres called the Black Hammer as shock troops to supplement orc regulars.
8. Orcus. My favorite demon-lord and probably the most recognizable D&D arch-villain. I never ran or played in a game in which the PCs encountered the Prince of the Undead, but I always liked the thought of him as being at the ultimate end of nefarious plots by evil cults and the like. Goat-headed and grossly fat with hooved feet, he looks like every demon should. And the Wand of Orcus is a great illustration.
7. Frost giant. As a fan of vikings, I always felt a strong attraction to these bearded, horn-helmeted, axe-wielding species of giant. I have a special fondness for G2: The Glacial Rift of the Frost Giant Jarl as a result, which also featured a white dragon and his mate, a white pudding (cool, who knew puddings came in different colors?) as well as the only (as far as I know) published appearance of the dreaded remorhaz (see no. 4 below).
6. Death knight. Fiend Folio gets a lot of flak from D&D fans, and I'll admit it does contain more than its fair share of ridiculous, essentially unusable creatures. But Fiend Folio also had some soaring heights of creativity, including one of my favorite monsters, the death knight. I always liked the fact that only 12 were known to exist, which, like the rare Type VI series of demon, forces the DM to get creative with their use. I used to have a list (long since gone) of names, personalities, and reasons why each of these former lawful-good paladins fell into evil. Armored in black plate-mail and riding on fire-hooved Nightmare steeds, death knights also make for a great visual image. And man, were they powerful--75% magic resistance (and if 11 or lower is rolled on percentage dice a magic spell is reflected back at the caster), plus once per day were able to use a power word of any type and generate a (20 dice!) fireball.
5. Dragon. Although dragons are the archetypal D&D monster, back when I was DMing I never used them much, probably because I was influenced by Tolkien and considered them to be exceedingly rare. I never saw the point in good dragons and prefer the standard chromatic evil dragons, including blue and black. But if I had to choose a favorite I'd go with the classic fire-breathing red, again, probably due to my love of Tolkien's portrayal of Smaug. I still love the mechanics of 1E breath weapon attacks, which did damage equal to the dragon's initial HP. You don't want to fail a save against an 88 HP ancient red dragon.
4. Remorhaz. Man, these things are nasty. A large remorhaz can swallow a PC whole on a roll of 20, and "any victim swallowed in this manner is instantly killed due to the intense heat in the monster's digestive system," according to the Monster Manual. If you touch a remorhaz's back, you take 10-100 damage (sadly I never had a PC do this, but as a DM I always wanted to break out the percentile dice to roll for damage).
3. Hobgoblin. I like orcs but they have too much of a Tolkien connection; Hobgoblins will always be more D&D to me. I fell in love with these creatures in part due to the illustration on the cover of The Keep on the Borderlands and they became my go-to monster for mass attacks. In my campaigns hobgoblins could whip up on orcs because they were more organized and took better care of their arms and armor.
2. Troll. I'm simultaneously repulsed and fascinated by the awful appearance of D&D-style/Poul Anderson-inspired trolls, with their mottled green skin, twisted, muscled limbs, wiry hair, broken teeth, and (worst of all) empty, black, shark-like eyes. Trolls' regenerative properties made for great fights, especially if the PCs fought more than one (it was always fun to have characters scrambling to apply torches to a downed but not destroyed troll as other characters desperately fought a second or third). There was also something chilling in these creatures' capacity to fight up to three opponents at once--I can picture very clearly a clawed arm independently moving to claw a PC at its flank or rear as the troll bites a PC standing at its front.
1. Lich. Outside of the gods or perhaps a handful of unique creatures like the tarrasque, was anything tougher in AD&D than a high level magic user, and, by extension, the lich? Access to a arsenal of powerful high-level spells is what makes this monster such a fearsome opponent. Combine the spell-casting power of an arch mage (18th-plus level MU) with an undead, magic-resistant body, and a supra-genius mind which has centuries or millennia to plan its schemes, and you've got a receipe for toughest creature in the game. I always thought that the lich encounter in Descent to the Depths of the Earth, if played correctly, could/should probably serve as a TPK. I mean, the thing had access to limited wish and time stop--enough said.