I voted for UT99, of course. I also think that UT 2004 is very worthy (Onslaught + Invasion-RPG mod) as is the original Unreal.
I disagree with Dark Pulse that UT3's game play is perfect. It's not bad and the movement has a very good feel to it, but the game play itself is far from perfect. You can't throw weapons and the piloting/driving of the vehicles just doesn't seem right for some reason and you can no longer do a first person view in them. (I can't put my finger on it, but something about the piloting/driving of the vehicles just doesn't feel right.) Also, as I understand it, you can't have pipe binds for weapons (ie, rocket launcher and AVRIL bound to the same key) or at least not very easily.
The vehicles take much more time to get used to. Much more time. There are certain things wrong with how the aim is vs where the hit actually results in certain situations. This is for all vehicles and turrets. Invisible things deflect your fire in certain situations. That is what bugs me the most. The no hit zone in certain situations.
The vehicles are much weaker and easier to take down than in UT2K4. First person is gone, I miss that in certain situations like when next to a tree in the tank and the leave block your view even though the tank is in front and aiming away from it.
The invisible wreckage that blocks you or that you drive up on or hit and roll on, ect. It stops you but not the incoming avril or weapons fire. Very frustrating!
Getting stuck to a wall or landscape feature or bouncing and turning 90 degrees on a small curb or some other nonsense.
FYI, pipe binds are pretty simple, but they do not work near as nice as they did in UT2K4.
This thread has the how to.
http://utforums.epicgames.com/showthread.php?t=594419
But, again, not as easy as UT2K4 and does not work as good. Many times you must press the key twice to switch so it's not nearly as fast.
I have my mouse #4 and #5 buttons on 3 way pipes. My center mouse has a double bind along side the scroll up and down funcion. Many other pipe binds as well.