Hi Diane!
Oh my this is a fun but tough decision since there are several great locations at Walt Disney World. My family loves milkshakes and certainly has their own opinions about which is the best.
My favorite location is probably at the
50's Primetime Cafe located in the Hollywood Studios Park. They offer some great flavors and they are nice and thick like my father used to make me. I love the atmosphere of this restaurant too as it takes me back to the days of growing up which of course ages me!
My next option is one out of a park and it is the
Beaches & Cream Soda Shop located at the Yacht and Beach Resort. This is a small quaint authentic little soda shop that offers everything great with ice cream. The shakes and malts are just some of the offerings with the big treat being their "Kitchen Sink". This is a massive sundae topped with everything you can imagine.
My final vote for a great milkshake while vacationing at Walt Disney World would be the
Plaza Restaurant located in the Magic Kingdom at the end of Main Street. This is a great place to get a burger or sandwich and top it off with an ice cream treat.
All three of these locations will accept reservations and it is highly suggested. If you want to try a walk up then Beaches and Cream is your best choice and during off hours. I hope you find your best spot for a cold treat during your vacation!