The best Sega Saturn games are Radiant Silvergun, Nights, and the Panzer Dragoon games. Panzer Dragoon, Panzer Dragoon Zwei, and Panzer Dragoon Saga. The first one is actually unlockable on the Xbox Panzer Dragoon Orta game.

If they're going to put one on the VC it should be Radiant Silvergun since it is so expensive and was never released in the US. However, I've heard it's more likely to appear on Xbox Live Arcade. The big CD based games are making appearances on Xbox Live. Symphony of the Night is downloadable on that service along with Rez, the Dreamcast & PS2 rail shooter that's similar in some ways to Panzer Dragoon's rail shooter design. Looks very unique, though.

From what I'm seeing the CD based games they're going to release on the VC are able to be ported because the CDs were used more for audio than filling them up with graphics. You can get that CD audio, convert it to a compressed format, and put it into a file that can fit on the VC.