Personally, the only parks I enjoy visiting during EMH are the MK and AK.
MGM Studios only has a few attractions open, and they're scattered: Muppets, Star Tours, ToT and RnRC I believe are the biggies. Since I'll be travelling with E-Ride cowards this year, the only thing we'd enjoy that 1st hour would be Muppets, so it's not worth it for us to go in early.
Epcot only has a few open too, and the only one we feel compelled to do early (as it gets crowded and FP's run out later) is Test Track. Mission:Space may be added to the EMH list, but again, we're cowards!
MK has the most to do, and it has the greatest variety of attractions open. If an EMH day is overly crowded, head out fairly early to another park and come back another time. Chances are you can still get a lot done in the first hour or two at the MK if you get in an hour earlier.
The only reason I like the AK for early entry is that the animals are
typically (no guarantee) more active in the early morning hours, so Kilmanjaro Safaris can be pretty fun at 8 a.m. I wish Dinosaur were open during EMH though.
Hope that helps!
Jon