Baby It's Cold Outside is also lovely. And I pretty much usually skip the Glee music numbers, but those two I've seen on youtube several dozen times.

Edit: So I just watched Teenage Dream again, and ~sigh~ Blaine is so dreamy, that performance is one effective seduction piece. Now, if Glee can do that in the middle of the craptitude that it's been ever since the 1st half of the season, might they do it again? That is the only reason I still keep it with the show somewhat. Cute gay boys. They make everything better.