Either watching, playing or both it doesn't matter. l think i am going to be in the majority when i say for me it has to be Association football (soccer) because there is no other sport that comes close to it in terms of passion and ferocity, it really is in a league of it's own. Now there are probably over 50 sports out there so i will only include the main ones in the poll so you'll have choose other if you're into less popular sports like Athletics,cue sports or aquatic sports etc.

Playing: Badminton
Watching: Boxing, gymnastics.
Esport (watching): League of legends.

Futbol to play, League of Legends (Esport if it counts) to watch.

Futbol to watch, Tennis to play. But to be honest, I like to watch any and every sport.
And since others are including eSports, I only enjoy Smash Bros. but might watch other fighters every once in a while.

Basketball to play
eSports Street Fighter EVO to watch

I enjoy watching/practicing martial arts, but I know that when I'll grow older IF I'm rich enough I'll play golf every single day...

Playing and watching both basketball and boxing. Boxing is rather recent for me, but basketball has been my favorite since I was a kid.

As a typical Canadian, hockey's my favourite to watch.

To play, basketball.

To Watch: F1 (motorsport in general, if I had more time), Snooker
To Play: Pool

If we're including eSports I do like watching people play Rocket League, I can't watch for very long, not that it's bad or I feel bad, but I just want to be playing myself so I boot up the game.

Hockey: Go Wings
Unless WWE counts then that.

Football is pretty cool too. Go Ravens, Cowboys suck wangs.