Like many people, the first park we visited was MK. These days, it tends to be Epcot and my girls give me a hard time if I try to sneak a different park into the itinerary for our first day. If you're staying offsite, though, you should choose any park which isn't the designated Early Entry park for Disney resort guests on that particular day. What day do you arrive and where are you staying?
Whichever park you visit you won't be disappointed.