Well, since I play in the servers belonging to the land of the FREE AND BRAVE, I can only answer for that.
Archer: Probably flying_dildos. Only archer I know off that can genuinely piss me off.
Polearm : Mmmmmm, not sure about that. Rhaelys is probably whom I would pick.
2h : Tons and tons of extremely good players to pick from but the clear winner is....well, saulcanner. That dude.
Mauler : *ahem*.....well.......honestly....not that many maulers in NA and from what I can see I well.....you know what? I will skip this one.
Crossbower : Rohypnol when he was that.
Shielder : Probably maple_syrup. Not the best 1v1 but damn, she knows how to handle bad situations.
Horse archer : No idea.
Thrower : Chucky_tosser all the way
Cav : Huey
LORDARK GOT THIS NAB THOUGH AHUEHEUEHUHEUE
Xbower+ cav : Rohypnol, when he was that.
Rohypnol is also an excellent 1h and an excellent mauler, but he doesn't work too well in group situations. He is more natural in places where individual skill is more valued, like horse xbowman or simple xbowman.