While I expect many today to be singing the praises of Beholders and Mind Flayers, allow me to show my love for the humble…
I still have a faint memory of the first time I read about the Rust Monster. It had to have been in my Moldvay Basic Set. Immediately I realized that this was an indirect way to to attack a character, other than just doing HP damage. However it was just such a CREATIVE idea to my young mind that it formed part of my early impression of what D&D was.
I think I read a letter or article in Dragon Magazine mentioning the hilarious incident of a party encountering a Rust Monster. Then of course about 1984 I started getting AD&D books and saw the classic illustration linked above.
An encounter with a Rust Monster is more of a puzzle than something that can be solved by simple combat. Can we find a way to hurt it? Can we manage to run away? Can we drop some stuff so we can run faster? Can we toss it a couple spare weapons to distract it?
D&D to me is more than just about hack & slash, it should require creative THINKING, and it is the DM’s responsibility to include some encounters and situations that are best solved with thinking instead of just rolling a d20 “to hit” roll.