I own Advance Wars, Age of Empires, Animal Crossing, Brain Age, Castlevania, Kirby, Mario 64, Mario & Luigi, Mariokart, Meteos, New Super Mario Bros., Phoenix Wright, Sonic Ruish, Tetris DS, Viewtiful Joe, and Yoshi Touch n Go.
I only regret getting Viewtiful Joe (which plays badly) and Yoshi Touch n Go (which I wore out of quickly). Age of Empires is nice and complex, but isn't as enjoyable as Advance Wars on a presentation level. Phoenix Wright is fun and funny, but seems to only hold up to one playthrough; unless you're the type that likes to read books over and over. I'm not sure what I'd pick between Tetris and Meteos, but currently I've played Tetris more often because the Yoshi's Cookie mode is a great brain teaser. Sonic has been fun for the most part, but I'm currently stuck on a tough level. Brain Age and Animal Crossing are great, but they're definitely acquired tastes. Mariokart is great if you love Mariokart or non-simulation racing games. I haven't played the New Mario Bros. enough in order to form a complete opinion on it, but I've loved it so far. Kirby has been great fun, but it's a little tough here and there. Mario 64 is great if you haven't played the original on N64. Mario & Luigi was short and sweet, so it might be more of a week long rental or a "buy and immediately sell".
My highest recommendations are Castlevania and Advance Wars, both of which I've put over 40 hours into, and I plan on putting in much more. Neither have gotten boring yet.