I think polls like this are going to usually depend on what game you entered the series with, since this is going to be the game that you grow to love dearly, and any game that deviates from this is going to feel like it's 'loosing what made the original so great'.
For myself, and probably many of the longer-running UT fans, it's going to be the original UT by a long shot. I'm no hater of newer UT's - I play UT2k4 and UT3 a fair bit (when I can be bothered to boot into Windows), but they're all great on their own merits, not as sequels to UT.
The original UT is still my favourite FPS of all time. There has been no game to replace it, and I play it daily. No game plays like it, no game manages to capture the sheer style in my eyes, every single damn level is dear to me, it was the last FPS to have truly memorable music. Irreplaceable.
Then again, I know people who started with 2k3/4, and honestly don't see why I make such a fuss about the original UT. This is simply because everything I mentioned in the previous paragraph about the original UT, they can apply to 2k3/4. The best game is the one you hold dear.