Sinnoh > Kanto > Gen VI (that may change depending of the evos) > Unova > Johto > Hoenn.
Sinnoh had the best starters. All of them have nice designs, origins, type combinations and are useful in some way.
Hoenn had the worst. Blaziken is meh and Sceptile is just... ugly. The only salvation is Swampert, who looks pretty decent and has a good type combination.