I don't know if it's a must, I think all consoles have a library that can be good enough to own just that console.
Wii will give you a whole new gaming experience, focusses on fun and creativity (like LostWinds) and brought back the lightgun games!!! This is Wii's gen. A must own imo.
Also for the more traditional gamer one of the HD consoles is a must own. I'm a big Resident Evil fan and after seeing the trailer of 5 I can't imagine missing out on that. Also some of the big RPG's and shooters will be pretty much HD-exclusive.
Owning PSWii or Wii60 pretty much gives you the whole spectrum of gaming.