There are two players I always enjo going 1v1: Ulti and Joker. I havent seen Ulti in a while (unless hes alias wh0rzing) but Joker is still around so out of them two, based on how much time I play against them at recent times, would be Joker.
Both of them have good aim but both, IMO, are not great at using the map surroundings to their advantage (SK/ambush around the corners does not count). I could be wrong but from my point of view thats my opinion.
Here is a list of other decent/good aimers I have come up against (in no particular order): G-Train, Trigger Happy, Frogma, nelots, blusky, DV8, Prince and prolly few others i forgot their nicks lol.
There is one nomination we have forgotten...
Best strafer... person who uses strafing techniques to annoy the f**k out of the enemy and making it hard to aim for the head.