Basketball. With other sports there are technical limitations. But with basketball the rules themselves are just so difficult to enforce (aka a ****ty rulebook that needs to revised to be more objective).
Definitely hockey... imagine trying to seperate 6'9" Zdeno Chara from 6'8" Hal Gill. Fukk that sh it.
Soccer would be tough too. Only 1 ref, running all over the place, trying to see when players actually get fouled or if they flop, especially inside the 18, would be tough as fuuuuu. Add in the fact that the fans will literally hunt you down if you blow a call, and I would say that its pretty tough.