Favorite Monster Overall
I’m tempted to say the 5e flumph, because the 5e flumph is really cool. When the Monster Manual finally got announced and I was free to talk in generic non-NDA-breaking terms about the work I’d done editing it, the thing I found myself saying most often was, “The flumph is going to blow your mind because it’s now really cool!”, and people were all, like, “Whaaah???”, and I was all, like, “Uh huh!”
I’m going to go for the obvious and say mind flayer. Because even though many monsters have risen to the heights of awesomeness in one edition of D&D or another, the illithids are the only monster I can think of to have hit that mark in every single edition. Right from their starting point in AD&D, mind flayers have exemplified a kind of evil that breaks the alignment system. They were the early epitome of the thinking monster — not just a deadly threat in the dungeon, but hatching plots and plans that could connect adventures and fuel a lifetime’s worth of campaigns.
As a player and DM, I’ve seen plenty of characters who weren’t afraid to die. I’ve played characters who could stand up to dragons. I’ve played alongside and DMed for characters willing to throw themselves into fights against demigods and titans without hesitation. But everybody who knows what a mind flayer is knows to be afraid of mind flayers, and I like that a lot.
(Pro tip: When the mind flayers first appear, playing the music from the Prometheus trailer can really help set the mood. You’re welcome.)