OK, I give in. Here are my favorites:
10. Wererats (MM) - What dank sewer system doesn't feature these sneaky, clever creatures? Part of my love for this creature is a fragmentary memory of an old horror comic book that featured wererats as the primary foes. Creepy!
9. Meazels (FF) - Secretive, sneaky predators suitable for any dungeon. Meazels were one of my favorite 'smart' dungeon dwellers. While the rest of the monsters were getting slaughtered the Meazels hung out in the background, picking off strays and feasting on the fallen.
8. Zombie (MM) - Sure they're slow and weak, but adventurers tend to provide the fodder for creating more, so what's not to love?
7. Kobolds (MM) - I always played kobolds as smart trappers and tricksters, taking advantage of their diminutive size to lead adventurers into dangerous traps and confining locations. Parties quickly learned that following a fleeing kobold was always a bad idea.
6. Lizardmen (MM) - A favorite jungle foe, lizardmen plagued many an expedition traveling by boat. Smart enough to use good tactics, tough enough to threaten even a prepared group, lizardmen are the ideal jungle foe.
5. Wights (MM) - I admit I never used 'real' level-draining wights. Draining undead drained stats instead. Still the wight was a personal favorite, mostly because of the excellent illustration in the original Monster Manual.
4. Dopplegangers (MM) - Dopplegangers are a tough creature to run well, especially with paranoid players. I always liked using them against friends and allies instead. When the village smith is actually a doppleganger all sorts of fun ensues.
3. Orcs (MM) - Sure orcs are fodder, but they're great fodder. Tough, organized, smart, willing to work for coppers, what evil overlord wouldn't love to have them in their employ?
2. Trolls (MM) - Original D&D trolls were just plain mean. Multiple attacks, regeneration and lots of hit dice. A great heavy hitter, and useful in most any environment.
1. Yuan-ti (MM2) - Genius serpent-men. Magic abilities. Psionics. Multiple forms. Why not use corrupt and tainted snake-men? Yuan-ti are a great addition to any tropical setting as inhabitants of an evil temple, the driving force behind a cult, or the secret powers behind the throne.
Totals: FF - 1, MM - 8, MM2 - 1