The Napa Rose is considered the "best" restaurant at the Disneyland Resort, and ranks as one of the best restaurants in Southern California period. It's in the Grand Californian Hotel.
Steakhouse 55 at the Disneyland Hotel is also very good; swanky, themed to the 1950's with big pictures on the walls of Walt Disney noshing with old-time celebs at fancy Hollywood nightclubs in the 1940's and 50's, except with all the cigarettes and ashtrays airbrushed out.
If you are looking for something in the parks and are not a Club 33 member, then the Blue Bayou is widely considered the "best" restaurant inside Disneyland with a great menu and waterside tables located inside the Pirates of the Caribbean ride. Reservations are strongly recommended.
If you are visiting this summer, then all indications are that the Carthay Circle restaurant and lounge will set a new standard for theme park fare. It's supposed to be like the Napa Rose for service and food, but set in a 1920's themed very fancy environment inside the Carthay Circle Theater. There's a private membership club called the 1901 Lounge also located in the Carthay Circle Theater, but unless you are a Club 33 member it's off limits.
Al Lutz had a great write-up of what's expected at the Carthay Circle this summer, since Disney hasn't yet released many details. It's in his latest update from late February, under the "Dinner Theater" heading halfway through this update...
http://miceage.micechat.com/allutz/al022112a.htm