I'd have to put Kevin Costner at the top of my list. He was okay in "Dancing With Wolves" because his monotone voice worked well with the narrative style. Unfortunately he uses that same monotone voice for every role! It took me about three movies (I hate to admit that one of them was the dreaded "Robin Hood") but I finally gave up on him. Cute to look at but sometimes you need more than that to get through a two hour movie!
My husband has an aversion to Sam Neill with his wooden demeanor. When he heard Sam did a miniseries as Merlin he nearly gave himself a hernia laughing over the whole concept.