For me it's Castlevania and Metal Gear, love both of those series. I'm also a big fan of the Mega Man (Classic, X, Zero ZX), Final Fantasy, Zelda, Pokemon, Sonic, Super Mario and Donkey Kong Country series. But, Castlevania and Metal Gear I can pick up any one of those games and sink myself right back into it. I do like to do occasional timeline runs of the Metal Gear games, I haven't done one in a few years now though, but with the HD collection in hand now, I might start one up soon.