My first choice for a half day is MGM or AK. Either park is perfect for 1/2 day touring, only if you start no later than noon.
I'm assuming that you are flying the redeye into Orlando, and having made that trip many times I suggest doing Epcot in the evening and resting when you get in. It's nice to check into your resort, take a nap, go for a swim, shower and head to Epcot for Illuminations. Another good choice would be MK, only if it's an EMH night.
Have a great trip!