One thing I like about 5th gen graphics is that they sort of fixed an issue with the graphics from previous gens that bugged me: the proportions. It was just annoying to me seeing your supposedly 4 foot tall character walk through a cave entrance that's almost the exact same height and length! In BW I think most of that is fixed, except maybe the entrances to buildings, but it's not as annoying as the cave entrances were. I'm OCD don't judge me!
But aside from that, the 4th gen. graphics, especially HGSS, were very nice.
3rd gen. graphics were also nice in their own right. They were definitely a HUGE step up from 2nd gen graphics, and the new sprites were also nice for the most part.