For DS, get Dragon Quest lX, as it's one of the best games I've ever played. I just beat it, and there's still so much to do! Also, PH and ST if you don't already have them.
For GC, WW, obviously, but another one I'd recommend is Super Mario Sunshine and Pokemon XD Gale of Darkness. Great games, with great graphics.
Wii, get TP if you don't already. (I've never played the GC version, only the Wii) Also, I recommend Super Paper Mario, Smash Bros Brawl, and OoT and MM for Virtual Console. (Once again, if you don't have them already.)