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RNG should be removed entirely. Sleep, freeze and confusion also.
The Trubbish line is conceptually cool and executes the idea well.
Alola had an excellent roster of new mons, especially the Ultra Beasts. I'd go as far as to say it's one of the best gens overall in that department.
The "slowness" of DPP is one of the most overexaggerated issues in the series. Not saying it's not a flaw worth mentioning, but people treat it as being way worse than it really is. I barely even notice unless I'm switching over from another game, and even then it's a minor annoyance.
ORAS wasn't that bad. It has its issues like the massively reduced difficulty and not including the Battle Frontier (which was one of the biggest "Fuck you" moments in the series' history), but between Soaring, being able to use newer gen Pokémon, DexNav, and the ported XY features like Amie and the PSS, I think it added enough cool stuff to be worth playing.
In terms of actual art style and aesthetics, BW/BW2 are some of the most visually appealing games in the series. The checkerboard patterns in the menus, the character designs, the way the official art looked, stuff like that. They're held back due to the visuals being subpar from a technical standpoint (low resolution, bad animation tweening), but even then I still think the games have better visuals than most/all other games in the series.
Megas are conceptually perfectly fine. They're literally just alternate forms. The only issue was that some of them were broken and some didn't do a good job at buffing their respective Pokémon, but that's not an unfixable problem.