I've had that Pokemon Yellow cartridge since I was a child and the battery still hasn't required a change somehow. That's probably my most sentimental thing. Most expensive is probably that GB Player disc alongside the Gamecube & extension itself, apparently it's all like several hundred bucks alone now or some shit. In-box Black 2 & XD are probably the most valuable otherwise, they're really expensive for some reason. Same with Soul Silver, which even without the box like me, it's apparently like $100. The thing that surprised me the most, though, was looking up the price of Shadow-the-fucking-Hedgehog and seeing it go for $80 even without the case on Ebay, so I put it in this picture too.