For 3DS games, I'd say X/Y, because that's the direction Pokemon is moving. The OR/AS remakes are good too, but being remakes I feel like they're "dated" in some areas compared to X/Y, while being better in others.
For other games playable ON a 3DS (ie. including DS games), I would start with HeartGold/SoulSilver if possible.
If you have a Gameboy iteration, I'm a purist and still think Gens 1 and 2 are the absolute best.
Upcoming Games To Get
Definite: Kirby Star Allies (Switch), Mario Tennis Aces (Switch), Fire Emblem (Switch), Yoshi (Switch), Pokemon (Switch), Kingdom Hearts 3 (PS4), Monster Hunter World (PS4)
Considering: Fe (Switch), Donkey Kong Country Tropical Freeze (Switch), The World Ends With You (Switch), Ys VIII (Switch), Street Fighter V: Arcade Edition (PS4), Kingdom Hearts 2.8 Remix (PS4), The Last Guardian (PS4), Shadow of the Colossus HD (PS4), Anthem (PS4), Shenmue 3 (PS4), WiLD (PS4)
