My initial thought was EPCOT. See all the countrues, have a romantic meal somewhere like France or Mexico, see Illuminations and soak up the atmosphere. I think MK is a good choice too, but since EPCOT is much bigger, it feels more intimate. You don't feel like you're tripping over kids and characters as much at EPCOT as MK. It depends what you like though. Both places can be very romantic, especially at night.