It'd got to be something that defined the decade nd changes the way people view coasters, not just be the best coaster built.
For that reason I have two candidates. Xclelerator, the first Intamin Accelerator put launches in the spotlight and were the first really popular launch coasters for a while. Changed the view that most coasters will go up a l lift at the start. It began a world wide race for launch coasters, it shown the world they can be just as viable as conventional gravity coasters. The top hat has become one of the most famous and copied elements in the world, being built on so many of them, including TTD and KK.
Air. Air shown that bigger is not always better, even if the US flyers seem to disagree with that and the whole point of them. Air shown that rides don't need to be huge and forcefull for them to be popular. It was built in 2002 and still has probably the longest queues in the park, which says a lot. It pulls enough G to make it quite forcefull, but not too much to make it really intense, apart from the turn after the fly-to-lie, which recently is very strong. Like Xcelerator, it changed the way people think about rides and created a whole new type with many new possibilities that parks are really only just discovering, but will be explored in the next few years, with any luck.