I'd say for casual, softball is pretty laid-back. It's like the more easy-going version of America's past-time sport. People also enjoy playing spikeball.
Competitively, id say football and basketball. From personal experience, most people playing these are trying to relive their glory days, and ive seen a lot of fights come from it. Nonetheless, all sports can be fun to play depending on how competitive you are and who you are playing with. People who love "their" sport typically enjoy the game either way.