Our Wii always seems to be on Wii Fit for someone or another these days, so we have been playing our DS's to death recently instead.
Currently my favourite DS games are:
1) MySims - quite different from the Wii version, and beautiful to play. A real change for those of us that like a more family-friendly game.
2) 7 Wonders of the Ancient World - picked this up cheap and was quickly addicted to it. A simple puzzler where you complete stages to build each wonder of the world in turn, and see information about its origin onscreen. Cute graphics and atmospheric background music add to the fun factor. Strictly one for puzzle enthusiasts only.
3) Mario Kart - still has to be in there, though not played so often lately
4) Touchmaster - the game most often in my DS, since the day I bought it!
5) Sight Training - better than it sounds, though I MUST get my Eye Age down below the 50's!
I especially enjoy the Boxing game on there.