i'm surprised no one has mentioned sparring and parkour and tricking. they're both technically mods, and all three require good skill.
for parkour, xioi, swexx, and mack all come to mind for various reasons.
tricking i think shoddy, flash, glimpsed, meija and dahstevy were some of the better ones, for either flow or creativity.
sparring, eviltorified, haku, and swexx. i really don't know enough sparrers