For me, there is really one that annoys me above all others:
"Are you saved?"
Now, I know this is common vernacular for a lot of Christians, but let me explain why I hate it. First of all, my own personal belief is that we strive for salvation but can't know in this lifetime if we've got it, no matter how many personal dedications we make to God. God's the judge, he's the one who is going to tell me if I have been saved. So that's just my brand of Christianity, which is to say NOT the "saved/born again" variety.
Secondly, I see it as one of those questions that seeks to create an "us vs. them" kind of mentality. If someone says no, they the person asking is now in this position to "instruct" the "unsaved" person and I don't think it's up to that person to "instruct" the other since how does he/she even know if they're "saved" themselves?
Finally, it isn't a question that Christians asked each other for hundreds and hundreds of years. To me, someone re-invented the rules along the way, and to be "saved" means something very different from one denomination to the next. It is essentially asking someone "Are you a member of my denomination?"