Diamond: The Underground and Poke Radar were amazing but it was so [hecking] slow.
Platinum: Massive improvement on Diamond speed wise, and some good postgame content. I can't think of many cons though.
Soulsilver/Heartgold: Probably the most content in a single Pokemon game ever, and one of the best Safari Zones too. Cons are the poor level curve especially in Kanto.
White 1: One of the best stories in a pokemon game, but the all new regional dex was a bit confronting, and some of the designs are controversial.
Black 2: Huge Improvement over one, with the integration of previous Pokemon, and some of the more interesting features like the world tournament and hidden grottos were amazing.
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Sun: Best looking game yet, the island dex system was pretty interesting as well. I didn't like how HMs were handled, and I don't like the change to fishing mechanics either.