I would have to say Castle Crashers. I've highly enjoyed other games like GW 1/2, Catan, and others, but they've mainly been single-player deals for me. Castle Crashers provides for some great gaming with friends, plus includes additional things like Arena and All You Can Quaff. Overall, it's an excellent value for $15. I wouldn't have minded spending a good $30-40 on it (and I wasn't really sold on the game before I bought it).