>Gen 2
Weather
Steel + Dark typings
Hazards
Day night cycle
Special Split
Rematches
Genders
Breeding
Custom Pokeballs
Equippable items
Shiny pokemon
Battle Facility

>Gen 3
Abilities
Natures
Double battles
Upgraded pc storage system
Contests
IV system

>Gen 4
Phys/Spec split(!!!)
Wifi functionality
Limitless item bag

>Gen 5
Triple Battles
Infinite TMs
Hidden Abilities

>Gen 6
Fully 3D graphics
PSS (best online features in the series)
Pokemon Amie
Horde encounters (best EV training method in the series)
Drastically improved breeding mechanics
Character customization
PR videos
Fairy-type
Mega Evolution
Super Training
Riding Pokemon in the overworld

>Gen 7
Z moves
Regional Variants
Removal of HMs

>Gen 8
Dynamax/Gigantamax (best mechanic since phys/spec split)
Wild Area
Raid battles
Online Ranked
Being able to change natures.
Being able to pass egg moves without having to ditch your Pokemon.
Being able to use 26 vitamins for one stat rather than 10.
Being able to relearn moves for free instead of Luvdisc grinding.
Being able to access the PC anywhere and always being able to look through the PC for prompts that ask to select a Pokemon.
various other QoL additions