Gyna successfully defeating the King of Camelorn, who tried to murder her as a child in The Mystery of Talara.
Elsaf Erol's entire tale in Beggar, Thief, Warrior, King.
Vernaccus and Bourlor is one long setup for one of these. Long story short; an archer who never misses shoots an arrow at a daedra, who vanishes into Oblivion before it hits, the arrow instead burying itself into a tree. The archer loses his legendary status because he has missed a shot, and the daedra becomes famous for avoiding the arrow. Time passes, the archer dies, the daedra has shrines built in his honour. The shrines to a daedra who "can avoid anything" are a tempting target for vandals. The daedra shows up to chase some away from a shrine, only to be struck in the back of the head by a thrown rock. Shamed, he flees inside the shrine. Another rock hits the door as he enters, causing it to close and hit his back. There was a rusty arrowhead stuck in the door, which pierces his back. The very same one he supposedly avoided so long ago. The door had been made out of wood from the tree the arrow was buried in.
While also funny, Sheogorath's 'champion' (a tiny bird) managing to outwit and defeat Hircine's champion (A freaking daedroth with lycanthropy) during Sixteen Accords of Madness is really freaking awesome.
Turning on his heel, Sheogorath beckoned the miniscule songbird to perch atop his shoulder, and strolled down the mountain, making for the warm breezes and vibrant sunsets of the Abecean coast, whistling in tune with the tiniest champion in Tamriel.