Brad Pitt in “Meet Joe Black”. Although, he is a good actor, so that may have been the director’s problem.
Before I opened the thread I thought “Glen Campbell in True Grit”, but you beat me to it.
Kevin Costner in “Robin Hood: Prince of Thieves” was particularly bad as well.
Best part of that whole movie was watching Pitt get taken out by the bus at the beginning. In what was supposed to be a tragic, shocking moment, the whole theater erupted in laughter. I knew then it was only going to go downhill from there. It did.