Most popular? Football (soccer for those Americans who are to coddled)
My favorite? Hockey
But honestly, American Football? Large men in heaps of pads running for 6 seconds at a time and crying if they break a finger. In soccer (and I use this term to differentiate between the two), you break your nose, you get sent back out onto the field. You break it again, sure, we'll sub you, but that only leaves 2 more. My whole deal with the American football scene is that soccer players are considered field fairies by all football coaches from the middle school level and up. This is why soccer is not very big in the US. We are told that it is a pansy sport from a young age when, in fact, it is played with only shin guards as pads and it is a high-contact sport.
Sorry about my rant, hopefully others agree.