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Gen 4 had the ugliest graphics of any pokemon generation. Gen 3 before it had simple but colorful and attractive sprite based graphics, and gen 5 after it had a cool super pixelated pseudo 3D thing going, but it still kept the sharp pixely aesthetic.
Gen 4 on the other hand is just a clunky, boxy, less attractive version of gen 3. The next crack at the 3D, gen 6, actually took it all the way and provided a genuine 3D experience.
Granted, Platinum and HGSS were pretty good games. Just sort of ugly in my opinion. I think they're so forgettable because it was the smallest leap forward graphics and mechanics wise between generations up until 6-7. For the same reason, gen 7 probably won't be super memorable either.