Many an hour have I spent being blasted by my friend, Alon. I have been killed on a wide variety of games by Alon, but I had the most fun being blasted into smithereens in UT2004.
It's just pure first person shooter fun. It's easy to pick up as any FPS is, and its unique tactics will keep you from getting bored. Dodging rockets becomes just as essential a skill as shooting them and the various weapons never outstay their welcome. Almost each of the weapons has a special property one can master. The Pulse Rifle stands on top with its ability to shoot it's own laser blasts to create a powerful local explosion. But even using the Flack cannon fragmenting blasts to shoot around corners, requires a little expertise.
The game modes include all the classics with the much beloved Assault mode, which tasks you with varied objectives set in interesting and well crafted scenarios. Also worth a mention is Onslaught, in which two teams use a verity of vehicles to capture strategic points in certain patterns. An ability to add game changing modifiers and a large database of mods online, will keep the game fresh for months and possibly years.
But really, this is a game to play with friends. I wouldn't have had as much fun as I had were it not for Alon, Goldy, Danigman and Capone, my loyal backstabbing gaming companions. And I remember with great fondness the two years we spent blasting each-other to bits and spouting creative profanities into the air while UT2004 ran on every computer at the collage where we pretended to study.
Now, its relatively high system requirements (for its time) should be no hurdle even for old computers. And I dare say that it still looks gorgeous and sleek. But really, you'll spend too much time hunting for the Quad damage power-up while emptying magazine after magazine into your friend's avatars to worry about visuals.