Fav - Minotaurs, killing huge powerful bull men is awesome, and unique weapons can be made from their horns with some imagination
Worst - Spiders of any kind
Lamest - a solitary 1st level humanoid with a nasty streak
As DM
Fav - Yuan-Ti, love having a whole mission planned around infiltrating a tribe
Overpowered - A Djinn with a pet dragon, actually used this once, after which the players no longer wanted to be in my campaign
Lamest - In my experience, it was a young white dragon, simply because the player rolled 3 twenties in a row for the instant kill, certainly not as dramatic as it should have been at the end of a rather difficult quest