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Platinum and BW2 rematches (both normal and Challenge mode) are the best, mostly because they felt challenging. BW2 rematches wspecially, where everybody had 6 Pokemon, held items and diverse movepools.
But my favorites are Malva and Kahili. Malva feels like the most interesting E4 member, if only because she is a Team Flare member. I'm disappointed that we didn't get to see her before the League, though, because it would be surprising if we meet her before and learn that she's both a Team Flare and E4 member. She and Kahili are hot af, though.
I kind of wish E4 characters would get more attention in the games. Most of them are uninteresting and wasted potential.