Based on my experience, I would def go to Magic Kingdom, I went there when I had no children and now the fact that I have a two year old who loves the disney channels, I would def go there.........although you'll be spoilt for choice on what to bring back for your little one, I'm def taking my two year old to Magic Kingdom when he's old enough to appreciate it.
if you go to Epcot - make sure you do soaring, although q is always very long so be prepared, it is fantastic though, i would skip this park if I was going back again but it was good, I think islands of adventure woud have been a better choice for us.
Good luck with your decisions becuase there is so much to choose from, I agree with the other poster that 5 parks in 5 days is too much, you need a lot of energy, you can stay in the parks from morning til very late at night so they are well worth the money in my opinion!