Outta all the starters, I think Venusaur learns the best, Torterra doesn't fall that far behind. It learns a bunch Attack base attacks like Razor Leaf, Bite, Crunch and Earthquake...C where I'm comin' from?
But battling wise...Actually using a Venusaur would be da best pick outta all the grass starters. Firstly, it's part Poison and can get STAB from the Poison Type attacks it can use; which automatically gives it an advantage against the other starters.
Except for Torterra, but hit it with a powerful enough special attack; a stat in which Venusaur has a decent amount in, it can take it down.