Definitely Mimi's outside the park. Huge portions, reasonable prices and a nice atmosphere. You will walk right past it on your way to and from the parks.
Inside the parks, there are all sorts of counter service options. My families favorites are: Plaza Inn, great fried chicken, big portions that can be easily shared. They have a character meal here in the morning with Minnie and Friends that you do have to make reservations for. We also enjoy Ranch de Zocalo for Mexican food, and Redd Rockets for pizza, salad and pasta. There is a corn dog wagon at the end of Main Street, we have gotten corn dogs there while we see the parade. Coke Corner is also good for hot dogs and has a nice seating area outside. The Blue Ribbon Bakery on Main Street has great baked goods all day long and sandwiches at lunch and dinner.
In California Adventure, we like Taste Pilots Grill for burgers and ribs and Corn Dog Castle for corn dogs.
For full service meals, Blue Bayou is good, pricey but fun to experience due to basically being inside the Pirates Ride. We have frequently eaten at the Carnation Cafe on Main Street, it has soups, sandwiches and salads for lunch and except for the tip isn't much more expensive than counter service. Cafe Orleans is a new counter service that is in New Orleans Square. Those three are the only full service restaurants (except for the private Club 33) inside
Disneyland.
We also enjoy all the character meals. Goofy's Kitchen is our favorite for dinner (at the Disneyland Hotel) and we also enjoy the Lilo and Stitch breakfast at Paradise Pier. If you have princess fans, Ariels Grotto in DCA serves lunch and dinner with the princesses. And Minnie and Friends have breakfast each morning inside Disneyland. There is also the Storytellers Cafe in the Grand Californian that has breakfast with Chip, Dale and the Brother Bear bears (Koda and Kenai).
If you click on the brown dining link at the top of this page, you can see lots of information on dining at Disneyland, DCA, Downtown Disney and the resort hotels. And you cannot make reservations online, but rather can call up to 60days out to make "priority seatings". That number is: 714-781-3463.
Here is another great link for Disneyland dining information:
http://www.allearsnet.com/dlr/din/dining.htm
Good luck, have fun planning your trip!