Both my DDs count Epcot as their favourite park
They are now 7 and 10 and have always loved Epcot even on theor first trip when they were 3 and 6.
There are many things they love there, many of which are mentioned above but I think overall one of the reasons they love it there so much is because the entertainment is partly of their own making! You have to work a little to do some of Epcot, which makes it all the better, I think.
For example, in Innoventions they loved to "build" a robot online and then race it against other robots and finally "make" one for real with plastic robot parts (free!). There was lots to entertain them in Innoventions actually, with interactive games and experiences galore.
The KidCot Fuinstops are always a big hit as are the passports; they love talking to CMs in all the different countries, learn how to say "hello" in all the languages and to find out fun stuff. They also like looking round the shops at the unique purchases to that country, trying on the hats in Mexico and Norway etc

They love trying the food too
Other little gems are the fibre optic lights in the pavement in Future World, the water play areas, the self powered fan benches, the dancing fountains.....
And that's not even mentioning the rides!