Cardboard Box SCD games.
N64 is too easy - as soon as you post what you need, you'll hit your collection goal in 2 months, pretty common stuff.
Pinball on consoles and PC I simply don't get. I know some folks obviously do as they keep making the games, but I really don't understand it.
SCD Cardboard boxes because:
They do exist (as in you will acquire them) but they are not terribly common, particularly in good to very good shape. They would look really really nice on a shelf and that's important as there won't be a ton of play value. Its a collection you can complete and know its complete (vs, pinball which will continue to make new games for current gen consoles I assume).