I would say the Turbografx-CD add-on. I only own one game right now. But looking at the library, it has some of the best CD games of the era, even better than the Sega CD (obviously better than the Atari Jaguar CD) in my opinion. I'm bidding on Castlevania Dracula X right now. I hope to get it for a decent price.
The second peripheral I love is the NES Advantage. You know, the "arcade joystick" for the NES. It just felt really good in my hands and incredibly responsive. It also adds a new dimension and play style to all my favorite games for the system.