As mentioned many times before, the Grand Mariner slushie. Expensive, but very good.
A new favorite of mine is the Red Stag Lemonade at the America Pavillion in Epcot. Very good, not too harsh.
I have found the margaritas to be horrible. Maybe we got a bad one or two...but I'll never get one again.
As sad as it may sound, I like Labatt's on tap in Canada. It is very hard to find that on tap in my area, so it is a nice change of pace.
Someone asked about value. Well...if you're looking for the most "bang" for your buck....I'd say the drafts are your best choice.