Based on my personal experience which I will admit is recent:

Loremaster - none has troubled me since I got beyond newbie skills so I can't really comment, but Kodiak has to get my vote just for his gemquest defeat of Dunccan.

Knight - Maleki hands down is the best I have fought.

Bard - Maximillian although the cast is limited.

Thief - Narissa of course

Ranger - again, none has troubled me, and there have been none worth speaking of recently.

Seer - although traditionally the mage's nemesis, only Cordon ever made me really feel I never had a snowball's chance in hell. The others I all felt I got the better of in the end.

Sorcerer - difficult since there are none I have fought \"for real\" that are worth mentioning. Eldereth and Salvador are the best I have fought but they were only friendly challenges.

Mage - me

Circe

Written by my hand on the 9th of Agamnion, in the year 1197.