I've seen a lot of video on DVD, and some of the most stunning were either HD-DVD or gorgeously upconverted.
The BEST HD-DVD I've seen has to be "Meet Joe Black". I imagine the Blu-ray can match it, but I don't believe standard DVD ever will look as sharp, as deep and as spectacular as this film looks in high def.
Amazing standard DVDs, upconverted, include:
The Last Starfighter
South Pacific
Hello, Dolly!
Funny Girl
The Geraldine McEwan "Miss Marple" series, which I'm sure was filmed in high definition. The image is also anamorphic (16:9), which is a huge plus.
A.I.
And I'll concur with others over the Harry Potters and Lord of the Rings films.
Many of my videos look wonderful upconverted, although some decidedly don't measure up -- transfer-wise -- and will hopefully be remastered/re-release.