I agree with Richard, for Inverted coasters, it's gotta be the front, especially Dueling Dragons, Air, like he said is clearly the best in the back, with some pretty cool flytime over the last little hill.
Wooden coasters are just crazy in the back.
Middle is just if I can't be bothered to queue for longer, front and back are both the best, depending on the ride.
Rita was pretty cool in the back, but a bit rough.
I'll do most coasters at the front at least once, especially if I have a certain piece of equipment strapped to my hand...