Our best service in a single experience is V&A's. We've only done it once, but that was an incredible event, service and food were both fantastic. They are certainly in a class by themselves.
Our best overall service has been at Jiko, Narcoossee's, Cali Grill and Citrico's. We've had good and bad experiences at nearly every restaurant we've visited at WDW. The worst service we've gotten at a signature restaurant was at Yachtsman - two visits that we were pretty much ignored by our waiters, we won't return, since the food was average as well. Which is a shame, because our first visit there was great.
We had a funny experience at Flying Fish. We were seated next to the waiters' station, where they enter things into the computer, etc. So we were treated to quite the show while we waited for our food! And our waiter was a bit overbearing, trying to force us into items we didn't want - and he apparently thought he was part of our dinner party, going so far as to invite himself into a photo with us. Strange. But other than that, we've had mostly good service there as well.