I voted for Ariel's Grotto. It's my most eaten-at restaurant at any Disney park. I liked it a bit more a year or two ago though. The menu for all WoC packages seems to change somewhat often. I also like it a lot better during Standard Time season when the sun sets sooner and you can see the pier lit up mid-evening if you have an outside table. I liked Carthay Circle when they gave WoC vouchers for ordering an entrée + appetizer or dessert, but now they do the prix-fixed menu and it's significantly more expensive than Ariel's for dinner. When I eat at Carthay, it's usually not for the WoC package. They do offer lunch which is about the same price as Ariel's dinner, though, so that's something to consider. Trattoria's lunch is the cheapest package of the bunch, but dinner is the same price as Ariel's and I prefer the location and offerings of Ariel's a bit more. I'll eat just about anything on Ariel's and Carthay's menu. I don't go to Trattoria enough but I always figured it was the more "safe" option.
That said, my next WoC package will likely be Carthay. It really is a beautiful restaurant and I've been to Ariel's quite often. I need something different. I might even just have a regular dinner at Carthay and go for the WoC FP instead.
