I would say all of them are good
This is why.
I don't know how old you are but when i was about 5 pokemon Yellow came out, I was so excited about this since i never owned a pokemon game. Now look back at the Graphics on those games at the time, they were modern, now there ****. But back then they were epic, i bet 10 years from now you would say how **** pokemon HG/SS or even B/W were, but at the time they were good.
But to be the worst would be Pokemon Green since it was so bad that it didn't even get released in English
Pokemon Red/Blue: 1 English Game
Yellow: Pikachu follow you
Gold/Silver: Day and Night
Crystal: Moving pokemon
Ruby/Sapphire: Faster bikes, run function
Fire Red/Leaf Green/ Remake of a great game
Emerald: Bring back gif images on pokemon sprites
Diamond/Pearl: Wifi
Platinum: New old pokemon
Heart Gold/ Soul Silver: Long awaited remake, pokemon follow you again
Black/ White: Pokemon are always moving(in battle)
Hope this helped
Mip