The best steak at WDW is at Shula's, without a doubt. Tender, juicy, flavorful, and the restaurant provides excellent service.
Second place would be Yachstman Steakhouse. Still tender, somewhat juicy, somewhat flavorful, and still excellent service.
From there it gets mirky: Many of the signature restaurants often have a good steak on their menu, though not much in terms of variety of cuts. Also, as you'd expect, the steaks are often prepared in a manner consistent with the specific cuisine of that restaurant. California Grill, Citricos, etc. I've read glowing reviews of steaks at Maya Grill, Flying Fish and Coral Reef as well.
Concourse Steakhouse is good, but just good; not great. Outback Steakhouse quality, let's say. Doesn't disappoint, but doesn't wow ya' either.
A lot of people like Le Cellier, but I have tried the place twice, and both times it was simply bad: Dry, grizzly steaks, and that's even after sending them back to the kitchen because the first steak was dry and grizzly. I'd steer clear of it.