Note - This post is part of the 30 Day D&D Challenge.
Day 23 - Least Favorite Monster Overall
That's tough... I think even the silliest creatures in the monster manual can be awesome if the DM presents them well. My least favorites would be the ones that are more annoying than deadly. Creatures that only takes half damage, or regenerate as fast as you damage them, or that give players multiple status effects on every turn... especially creatures that keep me stunned round after round, preventing me from actually playing the game... those can make D&D less fun. But again, a good DM can make a difference.
Also annoying are creatures that have ridiculously high AC, though that's often a matter of the DM using inappropriate monsters for that party. That's another thing - any creatures can be annoying if they're too high level for the party at that time. One of the most annoying character deaths I've had was when we were ambushed by Bugbears. They won the initiative and dropped my Sorcerer before I even had a turn. I didn't make any mistakes that led to my death, and I had to sit and watch the entire encounter without being able to play. (And it was 4e, so you know the encounter took a while.) Unpredictable instant death does not make for a fun game.
Fluff-wise, I hate that there's metallic bulls called "Gorgons", while the creatures that should be called Gorgons are called "Medusas". And it strains my suspension of disbelief a bit that I can kill a gelatinous cube with arrows.