Most of the obvious ones have been said, so here's some others...
Professor Layton is great if you can be bothered to import it, though replay value is minimal.
Surprise vote for Metroid Prime: Pinball - I'm hooked on that at the moment.
Castlevania: Dawn of Sorrow is an awesome old school 2D platformer, absolutely brilliant.
Meteos is a great falling block type game, with an inventive mechanic.
For traditional falling blocks, Tetris DS is a very good version of the classic.
42 All Time Classics is a good variety of card games, board games, etc.
Already mentioned, but Zelda, Mario Kart, Mario Bros, EBA are all fantastic. I've been playing Mario Kart for about 2 years now and I'm not bored yet.