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2, 3, and 6.
2 had a weak plot and just piggybacked on 1 in just about every way.
3, while it had FRLG and muh Emerald, also had a lot of "lel random shapes" designs for the new Pokemon, no backwards compatibility, and Hoenn is dogshit.
6 is so handholdy and feels like the beta test for 3D. No heart. Also Hoenn is still dogshit.
1, 4, and 5 are the pinnacles of the series. Gen 7 is looking pretty great so far.