XBOX
The first contender was, as you all can guess, the original HALO. Just out of laziness I didn't use Halo 2, because they are essentially the same graphically except for a few updates in the newer version.
Halo ran quite nicely, actually. The widescreen mode did not "Stretch" the game whatsoever, and the GUI looked as sharp as ever even on the Plasma screen. All the colors and details were as sharp as on a standard screen, and did not come off as "Blurry" as some games tend to come off on a widescreen. Multiplayer was, of course, a pleasure to do, and the game still ran quite nicely.
The next contender was a staple of any fighter lover with an XBOX- Dead or Alive 3. DOA3 looked absolutely stunning on the widescreen, and I saw NOTHING wrong with the graphics. The option in the menu to change the cinematics to widescreen also made it a pleasure, making the whole game one big widescreen pleasure. The fighting seemed so much more detailed with the widescreen, as it "Fit" the "2D" aspect of the game.
DOA3 was extremely fun to play on the plasma screen, and this is a definate plus for the XBOX in the HD realm. Because the game scaled to the TV (and not the TV to the game), the game looked fantastic, and I actually felt like having the widescreen was of my advantage, as opposed to to my detriment. Great!