To be honest, you can start with any game.
So... any of the definitive versions, really
Gen 1: Fire Red or Leaf Green (Pokémon Let's Go! is a different beast)
Gen 2: Heart Gold or Soul Silver (technically a sequel to the Gen 1 story)
Gen 3: Emerald (GBA) or Omega Ruby or Alpha Sapphire (3DS)
Gen 4: Platinum (it fixes all of the problems that people have with Diamond and Pearl, namely, DP was sloooooow)
Gen 5: Black and Black 2 or White and White 2 (you can mix and match, though; the more important point is that Black 2 and White 2 are direct sequels to B/W, albeit with a different protagonist, you can actually transfer some data from Black/White for some optional stuff)
Gen 6: X or Y
Gen 7: Sun, Moon, Ultra Sun or Ultra Moon (Sun and Moon have a more focused story, though)