The most valuable... I'm guessing the Final Fantasy Vinyls box set comes to mind having sold for $350. But that's a box set, for a single album I'm guessing either the original numbered Final Fantasy VII having sold for $300 or Red Dead Redemption having sold for $250. Mind you that these are the highest sales prices for them all. They can occasionally be found at least just a little cheaper. Those are the ones I can think of from the top of my head. Maybe there's something else that's sold for more. Wouldn't surprise me
These can all be found on the second hand market, though. As for the rarest I'm gonna guess that it's some of the Japanese stuff that was released in the 80s. Not easy to find many of those in the West.
EDIT: Just realized that the SDCC version of the Battletoads OST has sold for up to $265 on eBay. That's absolutely crazy...