We have had three trips to Disney with a party of 15. That being said, some restaurants were better than others...
The buffets are always great because you don't have to worry about someone's food missing, a wrong order, etc. Crystal Palace only has smaller tables. Boma was able to sit us at a very long table. Ohana had us split in two large tables. Hoop De Doo had us at 3 (or 4) tables close together. Biergarden had us at 2 tables as well.
Breakfast meals seem to run smoother than dinner. Perhaps breakfast food is easier/faster to cook? Kouzzina was a great breakfast, very long drawn out dinner.
We always booked through grand gatherings phone number. Times were never really an issue for us. We were not able to get Le Cellier once. When DH and I went alone the following year I can guess why. Small!