I mostly don’t collect for value, so anything I have that’s “rare” is just through sheer good luck and the fact that I rarely sell anything off.
Unquestionably my rarest item is Clay Fighter Sculptor’s Cut for N64. Bought it directly from Blockbuster back when the local store was selling off their N64 rental inventory.
I don’t really own anything else considered super rare, just a handful of harder-to-find games for various consoles:
PS1 - Castlevania Chronicles, Lunar 2 Eternal Blue Complete with almost untouched Omake Box, Strider 2, Arc the Lad Collection
SNES - Mega Man 7 & X 1-3, Castlevania Dracula X, Ninja Gaiden Trilogy
Game Boy - Mega Man IV & V
Also, I own 11 old-school Nintendo Game & Watches from my childhood, including Zelda.
Only valuable item I recall selling is Fire Emblem Path of Radiance for GameCube, for $50. Kinda regret it but not really, since we needed the money at the time and I didn’t really like the game anyway.