It really depends what you're team's play style is going to be. Venusaur is a amazing Sub Seeder, and if you want a tanky wall go for him. Blastoise, while not having as much bulk as Venusaur still has a considerable amount and has the best mega evo out of the 3 to go with it.
Blastoise was very underwhelming even in it's mega form for me. So, if you have another water type in mind (let's face it, a water type is necessary for your party in game while grass type isn't), I'd go with Bulbasaur. But, I don't remember too much to choose from when it came to water types. Starmie maybe. I haven't tried Clawitzer yet.
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