I'd say BW1 because it's issues are more grounded and personal. Like yeah Cyrus wanted to become god and rewrite reality but that's a very obviously fantastical goal. Shit, fucking Bowser wanted to do that in the Galaxy games and no one would call those games dark. Ghetsis on the other hand planned to manipulate the citizens to give up their weapons (Pokemon) through propaganda and idol worship and then use them to seize control of Unova. To pull this off, he indoctrinates his child into a radical belief system, and rejects him once N accepts the player's way of thinking. This is the sort of shit that happens in the real world and to me that makes it feel a lot darker than some guy using a horse-dragon and a dick-dragon to become a god. TCPI must agree that BW1 was darker since they had to censor out Ghetsis explicitly stating that N was his son while they left DPPT's Galactic plot alone.
Not that BW1 is grimderp or deep or anything, it's still a pretty light-hearted game with a saturday morning cartoon-tier plot at the end of it all, it's just a tad darker than the other mainline Pokemon games.
