This question is tough, the grass starters are my favorites!
Venusaur is almost always my starter since it's my favorite grass pokemon in RBY.
It's one of those pokemon that has a jungle vibe along with a beautiful palette. However, though I love the design, it is a bit bulky for my personal taste. 8/10
Meganium gets alot of flack. I am not big on stats and competitive play. As far as the design, it's beautiful, though it is missing the same charm as Venusaur, despite being more my style. The yellow antenna's are also a nice touch. 8/10
Sceptile is the only starter that doesn't really give me a strong positive review. There is just something about the tree-tail that seems too unnatural unlike the other designs. Though, I will say I love the face and it's shape, as well as the leaves stemming from its forearms. Blaziken seems to overshadow Sceptile. 6/10
Torterra's design seems like it is based off the origin of the Earth story as well as the great flood stories from a few cultures, including native american. The earth growing from it's back could be said is a little too over designed, but generally, I love the idea and the grass/ground typing. 8/10
Serperior had one my favor with the reveal of Snivy, but its final form only reaffirmed my love for the line. I love the emperor-like pointed collar and the trademark smug upward nose shape of the face. This pokemon has the most personality beyond just a great design and palette. 9/10
I am leaving room for improvement. I am hoping Chespin's evo's earns a 10/10 from me!