That's a pretty subjective question. Saturday is the Early Entry day for MGM, so if you're worried about crowds, that's probably the one park worth avoiding. The Early Entry park tends to be busier overall as it draws somewhat larger crowds at opening.
If you're asking for a personal opinion, I always like to start with Epcot--particularly when touring with young children. To many people, the Magic Kingdom IS Disney. Once you've seen Cindarella's Castle and experienced everything that the MK has to offer, it's tough to not be bored by a more laid-back park like Epcot.
When you start with Epcot, the excitement level continues to build over the course of the trip rather than seeming somewhat anticlimactic.