I think the top fighters are decided by three things - ability to cure/heal, ability deal damage/inflict and creativity. Someone must be able to cure what they are being hit with, while staying a threat, but they simply can't stay at just that, or they will remain mediocre. They must also have creativity to set themselves apart.
A good example of this is either Maleki or Finbar. If you watch them fight another knight, you easily see who is the most creative. They make a good use of general skills, pulling attacks other knights don't think to use, which ultimately wins them the victory. In the end, creativity is what puts you that step ahead from others.
As for the best, I define my best by those most persistent, successful and frightening to fight. This does include a good bit of mental games - those people who mentally psyche you out before the fight even begin have an excellent advantage.
Sadly, there aren't many shining examples of many professions out and about. Mages and bards are one good example, but sorcerers and seers are quite passive right now as well. Knights and loremasters seem to be quite popular.
That makes choosing a top one quite hard. If pressed, I'd say Maleki, for win/loss record, and for creativity, but he is not as active as other knights, so it's hard to make a completely accurate judgement. Krill gets a mention for kills (he's quite scary/annoying at times), as does Dunccan, for sheer tankiness. No matter how low and messed up you get Dunccan, he's near impossible to get that final hit on.
For smaller knights, a mention should go out to Jaco, probably the best of his peer knights, doing very well with very little.
For loremasters, the toss up of course is between Zenichiro and Kodiak. I personally would say Kodiak has a slight edge on Zeni, he's much more effective with defence, curing and movement. Watching them spar would be interesting to see...
For small loremasters, nods go out to Flamestrike (he's gotten great at movement and defence), Zuul (wow, just wow) and Lucifer (annoyingly effective).
For rangers, Sajora and Artec are the only active fighting rangers, though Tim is becoming surprisingly decent. In the end, points would go to Artec, for being more creative and aggressive.
One fun thing to bear in mind is how skillsets balance out. As a thief, mages and sorcs are not nearly as hard as they are for other classes, so it would be interesting to see how others rate other professions. Perhaps some find the mage profession the hardest of all!
Written by my hand on the 27th of Mournsend, in the year 1154.