Proper Football (soccer) definitely. There is a reason why it's the most popular sport In the world. It's by far the most exciting to watch. Anything can happen at any moment and there are no time outs every 10 seconds like American sports.
Toss up between Football (the worldwide one, not the American one), but I also found some enjoyment watching baseball too. I say this because baseball is one sport I have fun watching even in the park if people are playing it and I don't even know them. If I walked by some kids shooting hoops that's a big snore for me. I like watching FIFA most for watching something on TV, but usually baseball if I go watch in person (for free). :)
It takes forever for anything to happen in Soccer. It's a just a little ball going back and forth and then maybe, just maybe a guy can get it near the goalie enough to maybe make a shot. Ice Hockey is super fast paced. Basketball is fast pace. Baseball is slow until the playoffs. Cricket is only popular in the commonwealth states. Soccer is popular in countries that have only Soccer as a sport or only a few competing sports. The U. S. has four major sports and then a ton of other leagues. The U. K. has what, like two leagues? Rugby and Soccer? Russia has a huge Hockey super league that spans various country's. American football can be boring due to all the commercial breaks but at least it is hard hitting.