Frankly, considering the original DS was barely more powerful than the N64 (somewhere between 64 and GC graphically), the best looking games on it were not 3D, polygonal games. But it did in fact have some truly gorgeous 2D games.

LovePlus

Suikoden Tierkreis takes the gold for me:

I couldn't take my eyes off the backgrounds of some areas. A shame the voicework was so wonky in the first half (had to be sped up to save space on the sd card, from what I understand).

Okamiden looks BEAUTIFUL. Ghost Trick too.

Pokemon HGSS/BW/B2W2.

The world ends with you.

Kingdom hearts 358/2 days.

Okamiden.

Probably some others I don't remember.

Sprung.

I really liked how good the port of Mario 64 was. And thinking how early it released in the DS' lifecycle...

People got REALLY creative with the graphics on the DS.

I'd like to nominate the likes of Okamiden, 4 Heroes of Light, and Wizard of Firetop Mountain.

Do I remember correctly that all (3d) DS games run at 60fps?