Jiko is my all time favorite signature dining. It has a wonderful atmosphere, incredible food beautifully presented, fantastic desserts (mmmm,pistachio creme brulee!), and a great wine list. As far as service, it's very good, but not the best I've had (I've been twice, same experience both times). The waiters were better at the food recommendations and knew less about the wine selections.
Artist Point is my second favorite. The food was very very good, well presented, although portions were on the small side. Atmosphere was very relaxed and comfortable...on the romantic side. Service was very good the one time I went, better than Jiko, but still I prefer Jiko's food and atmosphere, as well as location (I love visiting AKL).
California Grille I'd rate 3rd place. The food is good, and has a nice sushi selection. Its claim to fame, IMO, is the ability to view MK and if timed right, both Wishes and the Electrical Water Pageant. Atmosphere is LOUD. Service was better the first time I was there (in the quieter Wine Room section of the restaurant), with the staff making certain our food was timed perfectly with the fireworks. Second time I was there, service was pretty lax, and the waiter, although not rude, was definitely uninvolved. Service was a little slow, but I attribute that to the day (Christmas night).
Tied for 3rd is Hollywood Brown Derby. The food is usually very good, although I did have a tough steak one time. The service is iffy. One waiter I had was really off...forgot dishes, charged for things not ordered, messed up credits on the
DDP. The next time I went, though, the waitress was amazing, making suggestions that were well-received, attentive to empty glasses, well-timed courses...she got a very good tip. Any restaurant can have a server with an off day, and any restaurant can have an absolutely stellar server, so I rate this one tied for 3rd mostly because of atmosphere (okay, but not very relaxing, IMO), and that one tough steak. Otherwise, very good.
I do think that Disney's Signature Dining is pretty good, no matter what you choose.