We were just discussing this last night!
Best: 'Ohana hands down. It is our favorite restaurant in the entire world (not just Disney World!) We've often contemplated taking quick weekend trips to DW (and we're from Michigan), just to eat there when we start talking about it.
Worst: Biergarten. The kids didn't like it, I didn't like it, and my husband thought it was so-so, but agreed we wouldn't go there again.