I'm not sure what the number one coaster in this case would be, but I do know of a number of coasters that have pretty much single handedly skyrocketed a park's attendance. Here's the list.
Batman- SFGAm
Magnum XL-200- Cedarpoint
Millennium Force- Cedarpoint
Superman: The Escape- SFMM
Tower of Terror- Dreamworld Australia
Medusa- SFGAdv (although part of a year where many rides were added, this one easily brought in the most people)
That's the recent crop, if you want, just say so and I can give some oldeer ones that made a big difference.