As always, I seem to be late to the party. So here is my (belated) top ten monster list.
Drow
Dark Elves got a lot of bad press on the top ten lists, mostly because of backlash against Drizzt and the overexposure they get by Wizards of the Coast as a result. Regardless, I will admit I have a soft spot for them though, mostly because of my fondness for the G1, G2, G3, D1, D2, D3, and Q1 modules. Back then, the drow were a shocking reveal. After all, the players had read the Elf entry in the Monster Manual, and it said that dark elves were “only a legend”. As long as they are used in moderation, I feel they can still be a dangerous and intelligent foe for your group.
Mind Flayers
Another major player in the D1, D2, and D3 modules, I grew to love them despite the confusing psionic rules and complete rip-off of Lovecraft. I have been told I have a flair for horror in D&D. I also enjoy intelligent monsters who try to outwit rather than outfight the party. So obviously Mind Flayers are a good fit for me.
Vampires
I6: Ravenloft was my hands-down favorite 1E module. As a beginning DM, the tips provided on turning Strahd into a credible threat for the party were a real eye-opener. Plus, you can take almost everything I said about drow and mind flayers above and wrap them into these adaptable undead.
Shades
Shades in the original Monster Manual 2 were an early example of what we would now know as a “template” monster. This adaptability suited me well, and a Shade Assassin became a major reoccurring bad guy in one of my long-running campaigns. Ever since then I have had a soft spot for them.
Dragons
I can’t leave these guys off the list, since they are actually in the name of the game. Dragons were designed to give the players a big fight and boy could they deliver.
Pixies
Players in my game learned to fear pixies. Seriously, they started leaving pre-emptive offerings for them when they entered the forest.
Halflings
Despite having read The Hobbit, I always had difficulty reconciling the Lawful Good alignment of Halflings in the Monster Manual with their amazing talent for thievery. So in my games, Halflings had a carefully maintained veneer of respectability, which was a cover for horrific mafia-like activities. If you knew what was good for you, you did not enter the Halfling parts of town after dark.
Succubi
These temptress demons were a pretty potent weapon when aimed at a group of geeky high school kids. Plus they possessed the dreaded Energy Drain ability. So I used them a lot.
Rakshasa
Intelligent, cool looking, and capable of creating potent illusions on a whim! I had a lot of fun pitting these against the party.
Orcus
If the party has to fight a demon prince, it might as well be Orcus. Well-stocked with powerful undead and demon minions, a battle with Orcus was a great way to finish off any campaign.
And honorable mention goes to…
Humans
Many of my most memorable “monsters” were of the human variety.