Some times ago, I would've said "guns", but that was before I started dabbling in games such as Half-Life, Unreal Tournament, Mass Effect and the like which, well... have guns. Never underestimate the power of games with guns when you need to blow off some steam! Of course, magic holds a special place in my heart, ever since I enjoyed magic wands, chain lightning, and magic missiles (King's Quest series, Baldur's Gate).
And I don't play many multiplayer games (except a couple each year when we hold our LAN party), so if I really *HAD* to pick one, I'd pick multiplayer.