The best WDW sushi is at Kimono's in the Swan. Very good, very fresh. It's a lounge type of place, so not exactly a counter service, not exactly a fancy sit-down. They don't open til around 5:30pm tho. The sushi bar usually closes around 11, but if it's slow, they'll close earlier. I've never had a problem stopping in, but often I am in grabbing take-out & not trying to get a seat. I doubt they take any kind of reservations.
Second runner-up is California Grill, but it's harder to get into. Making an ADR is pretty much a necessity. If you get lucky, you might be able to do a walk-up to the bar, but you can get turned away from that also if it's full.
If you have a lunchtime sushi craving, you can hit Tempura Kiku or MatsaNoMa lounge in the Japan pavillion at Epcot's World Showcase. The sushi is fairly decent, although I think they do some of the rolls up in advance - they can be a tad dried out sometimes.