Actually the Original starters well always be the best.
Ehh...It's opinionated. People will argue with me if I say 3rd gen pokemon were the best, some say the originals were the best...but I think the true colors didn't come out until the gameboy advanced games came out...Going from 2 tone games to full color was kinda amazing...
ANYWAY! I personally think that the change from Gen 2 [Silver/Gold/Crystal] to Gen 3 [Ruby/Sapphire/Emerald] was the best in the series... I don't know, I prefer the 3rd Gen starters overall because of their learnset and blend with their types...
Sceptile was just flat out grass, which was disappointing.
Swampert was Water/Ground
Blaziken was Fire/Fighting
I really enjoyed Hoenn, a bit more than I would've admitted as a kid. Overall I think that the way they created 3rd gen was a bit more creative than others...
Beldum/Metang/Metagross is a good example of that.