Just from the 5e MM these ones come to my mind: Ankheg - Pretty basic, but grapple and burrow is an interesting combination. Goblinoids - Love the social structure between Goblins, Hobgoblins & Bugbears. Could run a whole campaign just with them. Cockatrice - Group fights a crazy bird, one turns to stone, now they're scared. Shadow Dragon - Love the image in the MM and the fantasy of spotting a flying shadow in the sky, being like : "Oh boy, there is a dragon. And OH BOY, he doesn't look normal!" Kuo-toa - What's not to love about crazy fish people. Mephits - Interesting effects for such low level. Mimic - They get annoying very fast. But every tiime a group fights their first is a special experience. Mind Flayer - Hyperintelligent alien tentacle monsters. Lovely! Rust Monster - Forces players to approach a fight differently. Scarecrow - The first time a character of mine went down it was against one of those. Loved the moment they stepped down from their stands. Cult Fanatic - NPC with the most usefull spell collection at their level. I wish a whole lot more NPCs were designed that practical.

Ah yeah, number 10. When you describe how your monk looks everything in the eye to gauge fighting ability after slipping into a dungeon room packed with hooded figures, and the DM says "Give me 15 constitution saves." Iconic.

Vargouilles are neat. I like to use them as a prelude to a demonic incursion.

Owl Bears are just Dire Platypuses.

What did you think of the book?