The best latte I discovered was in Epcot near the American Pavillion. They have outside vendors selling delicious fresh brewed coffee in stands marked "Espresso and Pastries". I had the best hazelnut latte there! These stands are also in AK.
Main Street Bakery has an amazing frozen latte that rivals a frappacino!
The Kona Coffee Bar at the Poly has wonderful coffee, but they have strange hours.
I agree with the lattes from the coffee stands in Epcot, they are not Starbucks (oh how I miss my venti quad non fat vanilla lattes when at WDW) but they are very good!