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Personally this is how I'd grade each of the games, Quality being not only the game mechanics (which on their own would make USUM the best games in the series) but also elements like the sensibility of the plot, the pacing, side activities, post-game and graphical-audio-visual quality, this is why I put the Gen IV, V and VI games at or exceeding the Gen VI games.
As for Heart & Soul vs. Riding the Wave, the games leaning more to the latter I feel more so were coasting on the successes of their predecessors or in the case of RGB failing to experiment significantly with the world they'd created, this is why Yellow, Crystal, ORAS, USUM and LGPE are all lower on the X-Axis than their originals since they were coasting off their counterparts without deviating significantly like FRLG, Emerald, Platinum and HGSS did.