In terms of Money, and how hard it is to find.
For me, one of my rarest games is
Skies of Arcadia: Legends which can go for wayyyy too much money these days. I also own the original for the Dreamcast, which goes for about £50 online.
Minish Cap is another game, which I've seen go for £50 online.
I also own 2 Special Pikachu Edition Game Boy Colors, which have gone pretty high online.
Also, Skyward Sword with the special Wii Controller bundle, which has gone to £50 these days.
I think that's about it, my collection is lacking a bit, but I have some other things. For one I own over 100 360 games, About 10 Dreamcast games, 15 PS2 and PS3 games, 11 3DS and 13 Vita Games, and the entire Halo collections (1, 2, 3, ODST, Wars, Reach, 4, Anniversary).
So yeah, It's not that great, but I still want to get a few more games, such as Shenmue for the DC, as well as Twin Snakes, Paper Mario TTYD, and Xenoblade Chronicles.