I choose rated "R" I didn't want to choose the not under 17, because there aren't much movies with that rating in the movies, so I think the "R" is the most popular, and they're cool movies anyways.
It varies a lot really, I watch a lot of different stuff. Anywhere between a PG and an 18 (UK ratings), I guess. Probably the majority would be a 15 or 18.
I don't ever pay attention to the ratings (unless there's a laundry list of things it has that rating for--see: Munich), but I'd say 90% of what I do end up watching is R.