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I prefer the old water colors because of how they tend to make everything bright and less intense on color. Plus, it's like some sort of nostalgia drug to me. Sugimori's art has gotten better over the years, though.
The ONLY difference between the old and new gens of Pokemon I notice is that new Pokemon have very bright, intense color schemes, while older ones were more washed-out and calm. If you took almost any Pokemon from a newer gen, washed out its colors a bit, and calmed its color scheme it would fit perfectly with gen I and II.