I'd go for something that's well established and therefore likely to be bug-free. UT2003 is superb in every respect. With UT2004 out soon (adding bigger maps and land and air vehicles) it should also compete well with the likes of BF1942. If you want mad fun flying and drivin and blowin stuff up, the BF1942 may be the best one out currently, but UT2003 also has some cool mods like:
Invasion - all players work together to kill off increasingly fast and numerous waves of monsters (Instagib is the best version of this)
Matrix - includes a bullet-time mutator that allows you to slow everything down, do matrix moves and see bullet trails in slow-mo exactly like in the movie...etc