I was going to say Disneyland's Matterhorn Bobsled (being the first of the modern tubular steel tracked coaters), but as someone said about Magnum and the 200' barrier, the same could have be said about the Matterhorn... someone would have probably invented the tubular track at some point in time.
I never even thought about the Racer at King's Island, but Coastersrz is correct... that started the "second golden age." With out that, who knows... the recent wooden coasters that we have seen may have never come to be... and the existing coasters at that time may not have remained... nor would have the few classics that have been moved from defunct parks.
On the other hand, there is the old Coney Island Cyclone. It has set the bar for what a wooden coaster should be... many copies have been attempted, but none of them have come to match this 1927 classic.
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I promise I won't go off on a rant about those @#$%^&*! Toboggan's again.