Being married to a sports junkie, I’ve seen lots of various sports live; I’m not interested in sports much, but I like watching lacrosse live. Hockey and baseball are okay, but lacrosse is FUN to watch live. Give it a try if you have a local team, Mnemosyne - it’s like hockey, except with the fun put back in it. 
Extremely close for me between football and ice hockey. NFL football might just edge out. If it were NCAA football vs ice hockey, hockey would win going away, though. I think I prefer hockey as a live spectator sport, though.
Baseball is easily number 1
Hockey comes in second, but those sports’ seasons dovetail nicely.
But I’d really be disappointed if I couldn’t turn on TV and find an Aussie football game or Irish hurling just for the sheer variety. ESPN used to air things like that as well as Canadian football, which I enjoy more than the U.S. variety (though I live in the U.S.), but alas they no longer have that wide variety of things. I suspect they’ve all gone to cable channels I don’t get.
I’m shocked at the lack of love for basketball. I guess a lot of people mildly like basketball, but not many consider it their favorite.
Actually basketball is my favorite sport to play, and for a long time, to watch as well - until Knicks tickets got so expensive, and the team so unrootable to me (not to mention lousy) for so long that I lost my bug.
I still enjoy watching even good pickup basketball games, but as a true fan spectator of pro teams (which is what I assume the OP was about) you really have to know the players not just on your team but their opponents in the same league as well, and I just don’t care enough about the NBA any more to be able to claim that.
Basketball is the Circus Peanuts o sport, yup.
I play lots of racketball, but it is rarely on TV. I do watch it on line when a tournament is on .
Ditto. I don’t even care for pro football. My second choice would be MMA (no need for boxing).