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If you want a chronological order, then it should be:
>FRLG
>Emerald - HGSS (not any specific order)
>Platinum
>BW
>BW 2
>XY - ORAS (not any specific order)
>SM
In terms of a nice begining, I'd say either XY for the dificulty and variety of Pokemon (with extras like Amie and customization) or BW for the straightforward region and a very nice story which can easily be extended with BW2.
Finally, if you're going for mere aesthethics, I'd say HGSS, ORAS and XY are the best options as you get colorful, varied areas with cute mons and not so hard to explore zones during main story.
I tried to show my ex gf the series with FRLG, but she told me areas like the Pokemon Mansion and Sliph Co. were a pain, as well as knowing how to deal with the guards for Celadon.
Emerald is great, too, but I don't feel it is the best to begin with. Same for Platinum and SM.
