Prime Time 50's cafe - I have only been able to get a walk-up ONCE if I sit at the bar (and I am by myself) for lunch. They are reserved always, even for lunch.
Yak and Yeti is getting very popular, since the word is getting out that the food is pretty good - it is a newer place, so you may not have a good chance getting a walk-up unless you go mid-afternoon.
Rainforest Cafe is another one that you could wait up to three hours for a table.
Like the PP stated, it would be easier to get a walk-up in places like Epcot where there are so many more restaurants/counter services. The Biergarten is also one of my favs.