Akalabeth (Apple II) - Very early CRPG
Panorama Toh (PC-88) - Very early JRPG
NWC Gold (NES) - Gotta complete my NES cart set, and this is of course necessary as everyone agrees.
Doom v1.1 5.25" floppy version - Not any earlier than the 3.5" floppy version but way cooler.
Donkey Kong red cabinet, converted from Radar Scope. I don't have room for this but if price/rarity don't matter, than space can't either.