QuakeWorld is my fave - great physics, great netcode, very fast gameplay, superb blend of skill and teamplay, and is unrivaled from a UI standpoint.
, too much nostalgia for DFLW 

due to every game being different and full of action & intense. The servers with music and sounds on are great too (http://lesverrues.verygames.net/) & the mavity gun ...... the best weapon ever made
