From the Wii perspective, here are the great games that I own.

Dragon Ball Z (for real. it is GREAT)

WarioWare: Smooth Moves (Rent it first, though. You can complete the single player pretty quickly, which unlocks multiplayer. There are some cool unlockables, but if you're not going to keep playing them, or the multiplayer, it may be more worthwhile to rent. I love it though, so I bought it.)

And, well, Zelda.

I also own:
Trauma Center (almost put it in 'Great' category, but it's not as good as the 3 I put there)
Elebits (fun, but not quite 'great')
Metal Slug (been a Slug fan for a while)
Super Swing Golf (hardly play)
Rayman (Ok, but now obsolete because of WarioWare)
Red Steel (gave it away)

Wii Sports is free with the US system, but depending on how it's priced i think it's a worthy buy if you have family you want to get into videogames. Making Miis is pretty cool, and if you have the internet you have the few internet-based channels (browser, weather, news) and of course the Virtual Console which is a whole other beast.

Seriously though, if you have a Wii, get Dragon Ball Z.