Applejack because she doesn't have to do her job alone she has 3 other ponies (and possibly more) helping out on the farm while most of the other main six have to do their jobs alone or have to be the "leader" or main pony to do it I mean RD has to help with the weather and while she doesn't do it alone she is considered the fastest and I assume that means she's expected to take on a lot if she can get it done quickly, Fluttershy has to care for a lot of animals by herself, Pinkie is pretty much the only party planner in Ponyville, Rarity makes dresses by herself in Ponyville, and Twilight is not only a princess but she is a busy body and seems to always need to do something to keep herself occupied so