All of my favorites have their pluses and minuses.
Raglan Road is probably my number one. The menu is large and full of fun and interesting items. I've yet to have a bad meal there. The downside is that it is incredibly loud. It's a raucous environment at the best of times, and when the live music is on and the dancers are performing, it can be downright cacophonous. An intimate meal it is not.
Portobello would be my second choice. It's one of the very few restaurants in the Disney bubble that does good Italian food. In fact, I'd say it's second (albeit a distant second) to only Il Mulino in that regard. Unlike Raglan Road, the atmosphere here is subdued. It is definitely possible to have a romantic evening out when dining at Portobello. The most egregious annoyance is that the tables, especially those in the middle of the dining room, are impossibly close together. I mean, they are bump-your-neighbor's-elbow close. If you can grab at perimeter table or a seat in the lounge, you will be much more comfortable.
My third pick would be Fulton's. It's one of only a handful of restaurants on the dining plan where you can get good seafood. It's one of only two or three restaurants on the dining plan where the seafood menu is extensive enough to offer real choice. While it's not the best seafood to be had in a Disney restaurant (that would be Flying Fish), nor the best seafood restaurant experience on property (that would be bluezoo, which is not on the dining plan), it's definitely a worthwhile option. My major gripe is that it is expensive, even for a Disney restaurant, and unlike a couple of the previous respondents, I do not think it is worth the cost. For just as much or even less, in some instances, you will have a better meal and experience at both of the other seafood restaurants I mentioned. With the limited time I have at Disney World, it's hard for me to justify the time and money on a meal that I could be putting to better use at a superior establishment.