Ooooh thats a toughie. Apart from Rugby League, i love them all.
Probably just American, closely followed by Union and Gaelic. Although Canadian is in many ways a slightly improved (and odd) version of American.
I like them all, including rugby league. How anyone can dislike league but not American football is beyond me, the principles are almost the same!
Personally I find Canadian the least appealing. For those that don't know, in Canada the end zones are twice is big and you only get 3 downs. Which for me makes it the equivalent of widening the goals in SOCCER in a feeble attempt to make it more exciting and calling it a different game.