I loved what they did with Assault in UT2004, the idea of narrative-based deathmatching was ambitious as hell (as evidenced by the paucity of maps) but created a peerless gameplay experience in my opinion. Just a shame that the public servers debased it into threadbare "race" maps and assorted bullshit, given how massively difficult it was to map for.
And generally speaking, UT2004, what a Dirk Diggler-sized package. I don't think there's ever been another game that offered so much out of the box. And it had an exceptionally deep, skill-intensive combat dynamic with the advanced movement system and varied arsenal.