>>33688928Gen III killed the franchise in terms of Pokemon design, and cohesiveness and integrity of the game world.
Gen IV killed the franchise in terms of good story modes.
At the same time, Gen III and Gen IV made the biggest additions to gameplay, introducing DVs, EVs, abilities and the physical/special split. Gen IV also expanded the movepool a lot.
They were bad in some respects, but improved upon the series in crucial areas pertaining to the very foundation of gameplay.
HG/SS are the culmination of all that's great about Pokemon. They combine the best elements of the previous games and express them in one.
Gen V did nothing for the series but adding pointless horde battles. It tried to emulate the greatness of the originials and ended up looking like a cheap knock-off.
Gen VI introduced mega evolutions, yet it too didn't contribute to gameplay on a substantial level.
This is the difference between the changes that were made in Gen III/IV and those that were made in Gen V/VI. While the former are of essential nature to the gameplay and ease its flow, the latter are just gimmicks.
Haven't played Gen VII. If any of you guys knows more than me, they should enlighten me.