You should go for Pokemon Black and white 2 and then X / Y.
Black and white 2 have a huge amount of pokemon and its more focused on the story.
I dont think there are any side quests in any pokemon game... But the post game of some games has to offer some additional stuff like the battle frontier in pokemon emerald. I think there is also one in platinum.
I would go for Black -> Black 2 -> X / Y
I dont really like black / white 1 because you can only catch the new pokemon, but the story in B / W 2 would be easier to understand if you played b / w 1.
You could also go for platinum, i think it has the biggest post game content and imo the most interesting story of any pokemon game. You should be able to get it really cheap by now.
If you're interested in Nintendo content and understand german, maybe you want to take a look at my youtube channel! :)
https://www.youtube.com/user/RealGamingExpert
