Popcorn of course, there's even a thread about popcorn machines here!
Snacking is all good but you gotta keep a healthy supply of napkins nearby so you don't get grease, cheetle, crumbs, etc. on your games. I have a bathroom in my basement too so I don't have to walk far to wash the food off my hands. It's easiest to snack between balls if you are playing a multiplayer game.