I like 50-68 round midcap mags best. I find they're an all-around good balance between fun and realism. They generally feed reliably, carry enough ammo so that you're not constantly reloading, but also force you to have some trigger discipline in a firefight. And I personally like having the challenge of having to reload mid firefight.
High caps are ok if you're plinking or a skirmish or something, but I wouldn't think of using them in a game.