2. Bill Murray
Bill Murray hasn't come out and said he completely regrets doing "Garfield," but in his short cameo in "Zombieland" he does make a joke about it being the one role he regrets. What Murray has said on the subject is that the only reason he even agreed to do the role was because Joel Cohen, writer of Toy Story, wrote the script. Maybe that doesn't count as full blown regret, but I think it's bubbling on the surface.