Mage/hider is about the worse to duel. 90ac/700-800hp/30+str/50+ab/80+spellmantles/26caster/uncanny/120+concentrate/combatcasting/epicspells
If used right, there isn't much that beats it. Thing that do beat it is a ranged 50+sr monk with 80ac, with uncanny or 110+spot.
This isn't exact a good build to do mage duels with either, it wins but hiding to avoid spells isn't something common for mage duels. You always can melee a pure mage vs mage with 50 ab, deals 80+ damage anyway. But I don't think you'll get much mages duels if you do that
If you want to make a living fortress you best make a 88+ac sr monk/2sd/1wiz with 120hide or so, can resist spells, has high ac/dam, and can use hips too. Bit more wizard allows dispels as extra, pretty good against buffers.
Those builds are about the strongest, but "best", I don't think that does exist in nwn, just like there isn't a "best" player.
-Mel