Gelatinous Cube of course! Even better if it has chunks of fruit and carrots in it.
Apologies to the cube, but black pudding is my personal favorite slime. When it comes to monsters as a whole… Hard to say, really. So many good options. Kobold are fun to run as a DM.
/image That’s not where I saw that going in my head.
Don’t forget trolls. No hit and run, those fuckers Regen.
What, no votes for the Beholder?
I unironically voted for the cube. Those things always gave me the heebiejeebies.
Having said that, my other favorite dreaded beast would definitely be the Rust Monster. Turning your best gear into poop since 1977.
Here’s a cute comic I found on the Rust Monster: http://rebloggy.com/post/art-cute-comic-fantasy-rpg-rust-monster-dungeons-and-dragons-d-d-roleplaying-gam/76230345158
I have to vote for the Rust Monster, especially when you have a DM who is stingy with the magic weapons.