For me, hands down UT games. The are always well thought out, pretty well balanced and push the new games foward in their thinking.
The #1 best feature for UT is plain and simple, modability. Anyone can create weapons and mods and do it realitvely easily. The engines are incredibly flexible..examples America's Army, GoW, Bioshock, Red Orchestra, etc...not to mention all of the great community mods and maps that are spit out by the 1000s from the genearl UT community.
It is very difficult to get bored with UT. If one game type or mod gets old, you have 100s of new ones to pick from.
And I will firmly disagree with the statements about counterstrike. The two games I play the most are UT and CSS. CSS does not even come close to the level of UT in terms of skill levels required. I don't generally rate myself at the games I play because I play for pure simple fun, but most say I am very good at both games. The point of that statement is that I am quite familiar with both games and how to play them well. With that said, UT requires far more skill than CSS. It took me a week or two to get the general hang of CSS, it took 6 months to get even slightly good at the UT games.
This is all about opinions, so I thought I would share mine 
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