I would say the Magic Kingdom because in my opinion, it's the original and best. I think it's the best one to 'introduce' you to WDW and also some of the best rides are there.
I always like to save the best for last, so I'd go to MK last. I would start with EPCOT, AK, MGM, then MK. Then again I just visit all the parks in one day when I go!
I've only been to WDW once, but I think I liked going to Magic Kigdom first a lot though because before I went, one of the only things I knew of was the Magical Castle.
Start with epcot first then AK then mgm then MK. We did this b/c we figured we would enjoy them more in that order so we could finish our holiday on a bigger note.