I would have to say Brent Spiner. He tried really hard to be a smart ass, but all he ever manages to be is just an ass.
As has been pointed out by others, sometimes the actors have to put up with fans that have no boundaries, or ask really dumb questions. Your post made me think of an example.
When we saw Spiner in San Francisco, he was great. We got to spend some personal time with him, and was just a normal guy.
When he got up on stage, he was funny, and engaged. All was going well.
Then, Q&A time (which always makes me cringe at these things). A guy walks up to the microphone and asks Spiner "Are you gay?" The audience immediately began booing. Spiner said "that's none of your business." The guy at the mic made some saracstic remark about the audience, and Spiner replied "yeah, you're really winning them over now."
They engaged in a little more back and forth before Spiner moved on. I just thought "what kind of jerk would ask a question like that?" I thought Spiner handled it fairly well, and it didn't seem to affect him afterward, but still...