I remember sitting in the back of my mom's car when I was a kid. It was a warm day, she had some appointment or something...but I had a copy of the the Monster Manual...it was the first time I REALLY looked at them all from cover to cover. This list was not developed through play (mostly) - it's just the ones I thought were the coolest.
1. Spectre: Brutal. 2 level drain (if I remember correctly), incorporeal. I always imagined the spectre to be some sort of undead nobility.
2. Demigorgon: I liked DCSIII and this was one of my favorite images in the whole book.
3. Catoblepas: Shit. I mean, who could really stand a CHANCE against this if they weren't expecting it? And who would EXPECT it?
4. The Giant Crayfish in Village of Hommlet. That dude knocked off so many characters before their adventures could even really start.
5. Rust Monster: Recent victim of editionitis apparently. Too bad.
6. Displacer Beast: Just weird. Tentacles on the back...
7. Brownie: My personal gateway into the Unseelie...not that I've ever met one.
8. Umber Hulk: Beetle-men are cool, and this one was a beefy monster.
9. Pit Fiend UGLY. Barbed Whips! Flames! Bat-wings! Those beady little eyes...
10. The Green Dragon: Seems like this was the red-headed stepchild of the dragon family, kinda puny compared to the rest, but always seemed somehow more practical, down to earth - I could imagine this guy's day-to-day, whereas I never could identify with the Reds, Bronzes, etc...(maybe it's weird to 'identify' with a pretend dragon - I've given up making excuses for such things long ago...)