I find the "back of the front car" rule to be true on most coasters. Car 5 on MF gives the most
sustained air
, but not the most intense. The front car is the best ride on the S:ROS coasters as well. The back has it's virtues, especially on the B&M hypers. The first drop in back of Raging Bull has the most "throw-you-out-of-the seat" feel I've ever experienced. But for the most pronounced, "push you up" air, it's the front on most rides for me.
But to answer the question; the original, and possibly the best ejector seat is the third seat in the first car on Magnum.
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Without upstops, airtime would have a whole new meaning!