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This, in my opinion, was the worst generation, but that doesn't make it bad. It's still enjoyable. My favorite is Platinum, with the same reasons stated before by other trainers.
Definitely Whitney because Milktank was one of the toughest Pokemon I've dealt with, especially since it restored it's health every other turn. Claire is a definite second because Dragon type Pokemon are still kind of hard to tackle with only lvl40ish Pokemon.