Personally, I think Katy is the best choice between her, Rihanna, and Coldplay. I'll go a little further and say my opinion is unbiased!
I'm a football fan myself (Go Ravens!) and I've seen the Super Bowl handle all kinds of different artists ever since I watched Super Bowl 38...although I missed that halftime show for some reason lol
Ciara hit it on point...Rihanna won't give us a performance or medley that isn't going to be explicit. I don't think she would want to clean her act up for something that big either, so unless middle fingers and f-bombs get permitted on TV that night...wouldn't go for it
As for Coldplay, I think they are a close second. They could make a huge splash if they want to do it...
Katy easily is best to me because she's a NOW artist, which is what most of the last few Super Bowl performers have been, with the only exception being Madonna, maybe? Other than that, she's got the potential to make a lasting effort on the big stage, regardless if half of the football fans go crying to the NFL that she isn't the type for a halftime show!!