For me, it's Charizard. Yeah it's overrated and and a pain to have as a starter because of the first two gyms, but it was my first Pokémon and it always had a cool design.
My favorite Water Starter is Swampert and my favorite Grass Starter use to be Sceptile, it probably still is the best Grass Starter ever, but I have grown very fond of my Torterra. It's such a beast and a great battler. It's speed sticks, but it makes up for it with great defense.

