I don't know enough about Mystara or Birthright to judge them, and I certainly dislike Dragonlance. Which leaves Greyhawk and the Forgotten Realms. I guess that, just from the concept, I like Greyhawk somewhat better, but the Forgotten Realms are the place of many fond memories, so I have to go with them.
There is nothing "medieval" about the Realms, though. They don't even have a sliver of a medieval feel to them.