I'm surprised no one has asked me this before. Seems like a no-brainer, right?
I'm definitely a shirts guy, because in most things, you're paying closer attention to the other team than yourself and your own team. This way, you get to see way more nudity.
Well, now I'm already second guessing myself. My first thought was something like volleyball or basketball where you're focused across the net or on your defensive assignment, but I suppose if you were playing, say, baseball, you'd be seeing your own team a lot more. And then there are sports like soccer or rugby where you're kinda seeing everyone the same amount.
I guess my answer is whatever maximizes my ass-seeing fun. I also think we should go shirts or skins on the bottom sometimes. It's a great way to see lots of bits, because t-shirts don't hide things very much, and because you have to tie them around your waist with those little arms they fall off all the time! Unless you try to make a diaper out of your shirt - and succeed - it's a win win in terms of exposed genitals.
My favorite thing is when I'm watching a sport or refereeing a sport, and they go skins-skins, because they're having so much fun. This can be confusing in some sports, but the trade-off is that people having that much don't care about the score anyway, and they bounce around a lot to boot. That's flappy dicks and slappy tits for the win!
Short Answer: Can't go much beyond that whole flappy-slappy thing, can we?