Definitely. DEFINITELY. Skeletons. They just keep buffing them and it is so annoying! They were fine before they started going into spam mode when you got close.
Either Mojang does something to make Skeletons less annoying, or they remove them. Though I do find them easy to ignore since I mostly play single-player, and when I play on single-player I usually play on peaceful. Only time I bother turning up the difficulty is when I am looking for a challenge, or when I am on a SMP server. Even then skeletons are still just a minor nitpick for me so I'm not bothered much by them.
Though when I am in a cave and a Skeleton drops down on me and shoots me with an arrow, and in turn knocks me off a ledge (doesn't matter how deep it is) or into lava I will go on a unstoppable rampage.