UT2k4 is an awesome LAN game, so is Call of Duty (1 or 2, either is fine) or one of the Battlefield games. However with CoD you'll want everybody to participate, since I recall there are no bots in that game.

I'd also go for an RTS game. I never liked RA2 until I played it with LAN, then it's surprisingly fun actually. Starcraft would also be good but I'm not sure a lot of people will be interested.