I'd say Florida Gulf Coast. They are the first 15th seed to make the Sweet 16. They will be the team that any 15th seed that upsets 2nd seed will be compared to.
I said Florida Gulf Coast because its never been done and because i think more people will remember FGCU made it to the sweet 16 than another middle seed making it to the Final Four.
Florida Gulf Coast, it was a tough choice though because Wichita State arguably gave the national champs their toughest challenge of the tournament(not saying Michigan was much easier though)