The title is kind of stating it right here. Lets compare and contrast.
Blastoise and Swampert
This is a tough one and I know many of you will say Swampert. Unfortunately for Kanto it will be Swampert and for me it is Swampert. No one uses Grass types really so it makes Swampert practically invincible in all the regions. Plus ice beam.
Sceptile and Venusaur
I've used both plenty of times and this one definitely is also tough. My Sceptile was really meh at the begining but once I migrated it to future generations it became a boss. My bulbusaur however was easy to train, and became strong very quickly if you knew what you were doing. For me hands down Venusaur, for the simple reason it learns cooler grass moves than sceptile. Sceptile does not learn Giga Drain at all. (disappointing)
Blaziken and Charizard
This is the hardest choice of my life right here it's like choosing between my kids haha. Blaziken is amazing but so is Charizard. When it comes right down to it they are evenly matched. Charizard is weak to Rock, Electricity and Water while Blaziken is weak to Flying, Rock, Ground, Water, and Psychic. Blaziken has a huge disadvantage there on weaknesses. Charizard could use an easy aerial ace and KO Blaziken dead. Blaziken really can't avoid that either which is disappointing.. Blaziken could use rock slide but so could Charizard. In the end Charizard won here. Blaziken isn't as amazing as hyped to be... Charizard could KO it anyday.