Mine has to be the fight versus the Cowled Wizards chief group (with a mage casting time stop and spell trap).
It lasted forever because, near the end, I pulled off a Wish to get all my spells back as if I had rested. I don't know why this single time she didn't escape through a contingency time stop (what the hell is that hack anyways?), but I could pull it off after like 25 minutes of spell clashing (love these kind o' fights
Another nice fight would have to be against the Twisted Rune in a legit fight. What is it? Well, you do not profit from the fact which opponents you are fighting. That includes pre-buffing, get anti-undead items, spell reflecting items, etc.
Just you and your normal, everyday spells, and quite nice a fight!
Attachment shows an example of such a fight. At least, the beginning part of it!
edit: I play a fighter/mage/thief level 40/30/40