Get F-zero above all else. It's the one game that makes a DD worth it. Doushin is fun, but you can play it in English via the PAL GCN port, Doushin 2 is mondo pricey and is crap, Golf is a fun game but way over-priced, all the Mario artist games are sort of fun to mess with and can be had for cheap, so they're worth it, as Sim City is, well, Sim City on a console.
F-Zero track building is a blast once you get the hang of it!