I have had a Chocolate Milkshake there and it was the best one I have ever had, so it's totally worth going there to try one.I would imagine Ghiardellis has excellent shakes too but of course that is not in the parks.
Most people have already put these, but 50's PT has the best Peanut Butter and Jelly shake! If you are looking for quick, you can go into "Dad's Bar" and order it to go. Beaches and Cream has great ones, but if you get chocolate, expect major rich chocolate!! MK also has smoothies in Tomorrowland that are quite tasty, in a little shop between Star Traders and Cosmic Rays.