My DS is by far my favorite system since the SNES, and probably even moreso, but I don't think I'd say any games are "must-own" yet except for Advance Wars and Animal Crossing. Games like Mario Kart, Castlevania, Meteos, etc. approach "must-own" territory, but aren't there.
But then, I'm harsh. I'd say there are only two must-own games in the entire GBA library, three in the GC library ("quality, not quantity" should have been the motto for the GC, not the N64), one in the PSX library, one on the GB, none on the GBC, none on the NES, two on the N64, two on the SNES, none on the PS2, and none on the X-Box. So it's not like the DS is in any kind of bad shape for my standards. Actually, it's way ahead of the game considering how early it is in its lifespan.