If you mean just the vanilla games, what are you wanting from them? Something to play through the story once and then put down, one to try to collect everything, one with an extensive post-game? There's plenty of good places to start.
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Scarlet
Scarlet
If you want a broad post-game, Pokemon Emerald might be the one to go for. If you have a good time with the main game, you have a huge battle facility to explore and try to conquer. It is some grinding to get a team to the point where they're viable, but that in itself can be rewardning too (you can also save edit if you're not prissy about it).