I say Big Macintosh.
True, AJ is most likely faster, but Big Mac is bigger and older. AJ only managed like half of the apple trees without his help and we all know how exhausted she was. She did last a while, and she's stubborn, so she's got that going for her. But, who ran the farm before AJ was old enough? Applejack wasn't always a grown mare (nor was he, but I guess they're parents took care of it before they presumably died--though I'm not sure if it's really confirmed) so Big Mac must have been completely in charge and maybe with the assistance of other ponies (my assumption is Caramel--I can imagine him being one of Mac's peers).
And remember how much he could haul in Hearts and Hooves Day? He's one dang athletic stallion. Yeah, that's a hauling kind of strength (as Silverhoof says), but endurance is something that comes with both his strength and bucking, I believe.
But you know, that is a good/tough question. I may be wrong and it's Applejack.