>>33706064imo play whatever, most of the changes past Gen 4 are just quality of life things. If you have a 3ds then XY is actually a good place to start.
Main things to note are:
-Gen 2 added breeding mechanics (two compatible Pokemon produce and egg of the female's species) and held items
-Gen 3 added Double Battles, Natures (+/- 10% in a stat), and Abilities (unique modifiers)
-Gen 4 added Physical/Special split (types are no longer all a specific type)
-Gen 5 added Triple/Rotation Battles (removed from Sun/Moon)
-Gen 6 added Mega Evolution (super forms via held item that gives +100 to Pokemon total stats and generally new ability)
-Gen 7 added Z-Moves (super moves for massive damage/utility via held crystal, they pierce Protect to deal 25% of damage)
