I love Unreal Tournament. I've been playing it since it first came out, and to me it beats Quake 3 (it's big rival) in every respect; replayability, maps, weapons, game modes, game mods (though admittedly after a while I stopped trying Q3 mods, so maybe later Q3 mods were better than UT's), everything. I still play it regularly, and I have a ton of maps installed, and lots of mods and mutators, such as XMaps, Soul Harvest, Weapons Factory, Unreal Fortress, etc.
I mostly play offline, as the bots are so great, but I have played online a few times. But I like to use various mods and mutators, which are almost impossible to find being used online, plus often the online players I encounter are *extremely* good, and
I didn't like Unreal Tournament 2003, nor did I like Unreal Championship (original XBox), which is what UT 2003 was based on. Unreal Tournament 2004 is a lot better, but not as good as the original UT, and I thought that Unreal Tournament 3 was very disappointing (and how could they even think that trying to add a real war story to a serious of arena matches would make sense?). Unreal Championship 2 (original XBox) is fantastic, it's nothing at all like he first Unreal Championship (nothing like any Unreal Tournament game, either), and really should have been ported to the PC. In some ways, I think that UC2 is the best Unreal game, but overall I'd say that the original Unreal Tournament is the best Unreal game.
One of my all time favorite too. I see it as the perfect FPS game. Never surpassed by any FPS game up to today.
On the PC, I'd agree. But to me Perfect Dark (N64, XBox 360) is the best arena based shooter (plus it has a real, and very good single player campaign). Note that I mean Perfect Dark, of course, and not Perfect Dark Zero.