I haven't seen them all, but I'd have to throw a vote out to Lion King for best looking 2D animation. I was completely blown away by the digital projection in theaters, and the quality of the Blu-ray. It's a reference-title for me when I was shopping for a new TV (as was Tron...)
It also wins for one of the best ever 2D-3D conversions I've ever seen (and I really don't buy into the whole 3D thing... Don't even have it at home, except for that Sony Personal Viewer thing... Neat gimmick, but it's still a gimmick to me.)
But I bet any of their 'catalog' traditionally animated features look incredible. Disney does not skimp on quality when it comes to those... (Princess & The Frog I don't think is hand-drawn. It's 2D, but I think even Disney largely does all of it on computers these days...)
CGI: Bolt or any of the Pixars...
And I'll agree with Tron: Legacy for live action.
Pick your favorite vault title, Pixar (or Bolt) film, and Tron and you can't go wrong...