This thread is all WWF/E and WCW, huh? I'm gonna say AJ Styles' reign as X-Division champion needs to be on this list. In fact, I think you could argue it should be number one. Not only did AJ build that belt from the ground up, he made it matter more than the WHC. You can talk about how the IC title was briefly more legitimate than the WHC, but the X-Division title was THE title in TNA. Not only that, but the "X-Division Style" influenced an entire generation of indie wrestlers.
We'll never see a mid-card title have that much influence again in wrestling history. Calling it.