Well that's easy...
Granted, it's hard to pick just one, but if I had to I would look no further than...
TEENAGERS FROM OUTER SPACE!
One of my all time favorite RPGs, especially with the modifications I made, TFOS is long overdue for a revival.
With the immense, and widespread popularity of Anime, and Manga in the USA, this game could be revised, expanded, and re-released with little effort. I would probably utilize all new art, hiring artists with a genuine grasp of the current Anime/Manga art styles.
I've discussed this game many times on the blog.
Notable entries include Direct Hits and Campaigns A-to-Z: Blast City Blues. Check'em out.
Man, oh man I would love to be in charge of a new edition of this game.