This one's really tough, because none are really consistently brilliant for me. The first Timesplitters didn't bother with a singleplayer, though the other two are fantastic, Goldeneye and Perfect dark are both stellar but in different series (and PDZ sucked), Unreal Tournament '99 and 2004 rocked the house but 2003 and 3 were a bit dissappointing, Battlefield 1942 was absolutely amazing but let down by every single battlefield game since, Operation Flashpoint 1 was wicked, but Dragon Rising not so much, Call of Duty has had it's ups and downs,
Out of all them, both Timesplitters and Unreal Tournament have two really good games with really good bots. I'm tempted to vote Timesplitters because UT has struck out more, but Timesplitters' bots are just a bit behind UT's ones.
I just can't choose- it's either Timesplitters or Unreal Tournament. Dammit it's too hard to choose!