Yellow: Perfect balance of crispness, minimalism and colors. Every town has it's own palette to give it extra character, it worked very well.
GSC: Extraordinary color palette with a very clever Day/Night/Morning system that looks amazing. I just wished the grass was less green. GSC maintained the integrity and minimalism off RBY, It's almost impossible to describe how good it looks on a small GBC-screen.
FR/LG: Tilesets looks sharp and the detail is quite stunning (marching through Mt. Moon had me realise that!). The color of the grass was a neat change. But it sadly suffers from a head-bigger-than-body syndrome. The sprites looks hideous and childish.
I haven't played anything ahead of Emerald, so can't comment about that. But when I saw the D/P graphics for the first time, I made a conscious decision to not spend anymore money on Pokemon and since then, I've been into a Dark Age.