For the competitive scene between 1-3 generations
1st gen Venusaur is the best
Blastoise is the worst with Czard inbetween
2nd gen
Meganium is crap
Feraligator is sometimes used in lower brackets
Typhlosion also bad
3rd gen
Swampert - is a god competitively
Blaiziken sucks machamp outclasses he's too vulnerable
Sceptile same as feraligator
As for non competitive it goes by difficulty while playing the game