I'm going to go out on a limb and say Iron Wolf replacing Z-Force. Yes, Goliath is a MUCH superior coaster to the both of them, but Z-Force's removal led to the start of B&M. IW was the first coaster by B&M, and when it opened, it was a damn good first coaster. Sure, it wasn't the tallest or fastest, but it led to other parks getting B&Ms, Great America calling B&M back to design the first Invert(speaking of which, Batman replacing Tidal Wave was also a good choice), Busch Gardens wanting them to design a sitdown which would be the father of the floorless, dive, hyper, and giga. Iron Wolf was a bad coaster, but it sure as hell was an important one.
I also think that Banshee replacing Son of Beast was a really good choice.