Split on this one.
Objectively, Epcot would be the best choice. It has a lot more to see and a new feeling since your last trip. However, you cannot really see it all in a single day. So, there is a chance that you will feel frustrated by not seeing everything. If you are okay with that, by all means go. As people have said, M:S, Soarin', TT, etc. are all great new additions. Also, there is nothing like an night spent around WS ending with Illuminations:ROE.
On the other hand, MK is a great park for a single day. It definitely has the Disney feeling, and, since you are seen most of it, you can pick your favorites.
The other two parks are great, but they just don't represent WDW as much to me.