OK, I was all set to say 'Gandalf', but then saw another possibility. But let's run em down:
Merlin: He's a pantywaste. No, really. The only reason he made the headlines is that he was the only wizard around at the time that could actually do magic. he could only do what little he did because of his bloodline. (half demon) The original tales all show him as a remnant of an older age: senile and unreliable. Low marks across the board except in a completely magic-deprived environment. Trapped in the end by an unremarkable elf maiden (Nimh/Nimue) while pursuing her with lust on his mind.
Gandalf: Personally sent by a major power as a member of a team of 5 whose mission was to thwart the largest evil power remaining in the universe. His council was considered worthwhile by Galadriel, the oldest being on the shores of Middle Earth save possibly Bombadil. At the height of his power, summoned and dismissed the his defrocked superior by sheer force of will, even though his opponent was still technically on his home turf. Personally stood in for the gates of Gondor against the entrance of the Witch King when the gates had been thrown down. A pyrric victory against a balrog is still a *victory* against a *balrog*.
Gargamel: Never heard of him. Ipso facto: not a biggie. [Looked him up: Some Smurf-related villain. Points for me for never having heard of him. Clearly not a real contender, even against Rincewind]
Elminster: OK, big and bad, but come on... one of many many powerful wizards in the world of Greyhawk. Not even clearly the most powerful ever in that world. Consider briefly an Elminster v Gandalf steel-cage deathmatch: Elminster casts, Gandalf saves. Elminster casts, Gandalf saves. Elminster casts, Gandalf saves. Repeat until all of Elminster's memorized spells are cast and his staff of the Archmage is out of charges. Gandalf breaks out Glamdring and cleaves Elminster in two while Elminster is still complaining that magic users can only use daggers.
Potter: Not bad. Clearly a standout on the planet: he can resist (even with help) the uncounterable death spell and summon a protective spirit essentially untrained. A human mage whose long term potential we do not really know yet. I kinda doubt we'll get something like I-died-and-was-personally-sent-back-by-God-to-fini sh-my-work though.
Rincewind: Full marks for having luck. Luck trumps a lot. Forty two ranks in Mastery of Cowardice. Fleeing a lot doesn't hurt in avoiding checks in the "L" column. Doesn't actually end up with any in the "W" column, either. Rincewind's score vs all other wizards: 0 wins, 0 losses, 6 byes.
Ged: Well dang. IMO an actual contender for Gandalf. Done the personal-purification-by-fire thing too. Conversation considered worthwhile by Orm Embar, the rough equivalent of Ancalagon the Black, possibly even Illuvatar. Has Gandalf talked to Illuvatar or Ancalagon? I don't *think* so. One wouldn't give him the time of day, the other would consider him a light snack. On the other hand, Ged at his zenith was still beat down by a bunch of invisible evil spirits, albeit in their own temple. So he fails the Gandalf-summons-Saruman comparison.
Excel: Well, for sheer diabloic evil, Microsoft gets their toe in the door, and for being the greater evil force, trumps both Merlin (who was only half-demon) and Gargamel (who is evil by virtue of contact by smurfs). You can't run or hide from Microsoft either, so put a 'W' in the column 'v Rincewind'. But as Ged said: It is light that repels the dark! Light! Heavy, convincing losses against Gandalf, Elminster and Ged. Eventually driven Sauron-style out of Redmond by the Open Council, and relegated to History Channel documentaries.
I think the Ged v Gandalf fight would be the best match: I imagine it taking out one heck of a lot of real estate before it was done. Nothing short of a Covenant-style ritual of desecration is going to completely overthrow either one. Definitely worth $50 for the Pay-Per-View of that fight, particularly if they could get Madden and Miller to co-commentate:
Madden: Whoa! That had to hurt! Slobberknocker magic doesn't get any better than this!
Miller: Yeah the dwomercraft is really at a high pitch down there. As the sumerian god Enki once said to his consort Innana, just before he released the curse of Babel...