Vythonaut: i haven't had the chance to play UT3 though.
In my opinion the worst in the series but still a great game. Frankly the biggest problem was the boring style (which was more like the grey and "mature" Gears of War than the wacky and colorful UT 2004) and the removal of the double jump which which made the game a lot less fun and dynamic. The feeling of the guns also just wasn't as satisfying as in the earlier ones. I was very let down when I played it on release but I gave it another try a few years later and enjoyed the heck out of the campaign, didn't play it much in multiplayer, though (instantly got my ass handed to me whenever I tried it online while I generally did well in UT 2004).
It also sucks that the modding community for that one never quite took off, as far as I can tell, while there was a mindblowing amount of amazing user-generated content for UT and UT2003/2004. Not sure if it's because the game's fanbase was too small or because modding it was too hard or something.