Also worth considering if the outcome would be different if it were in a straight up arena type setting, or if the combatants were left loose in a large dungeon or something to seek eachother out.
It's really hard for me to gauge who my 'primary' character is, considering I've had so many in each game (except Arena) but I'll go with my character from each of the games that got the most powerful Here's a run-down of them:
In Arena.. I only ever mad one character, an Argonian, and he never made it out of prison. The framerate hurt my eyes. (it's fair to say he's just fodder in this fight)
In Daggerfall I had an awesome Breton Battlemage. Pretty good with a sword, but because he had spell absorption, any spell he cast that had a radius near him he absorbed, so he could pretty much infinitely (and incredibly rapidly) cast awesome huge spells.
In Morrowind I have a level 60-something Altmer mage who wears the most enchantable pieces of armor in the game, and rocks some pretty awesome stats boosted by said armor, spells, and potions, and he wields Stendarr's hammer on special occasions.
My Oblivion character, another Breton, is pretty balanced really and is good with Magic and bows/stealth but is best with melee combat. He has some pretty inventive spells and boosts, and lots of good reflect damage/spell potions.
Anyway, I think in a straight up Arena match my Morrowind character would win out, due to some amazing defenses, the ability to reflect spells like mad (which would probably instakill my Daggerfall character) and dish out giant balls of ludicrous amounts of damage.
If there were more room for stealth however, my Oblivion character would probably win, if he were able to sneak up and kill my daggerfall and morrowind characters before they had time to let a spell fly or quaf any potions.