Not pointing out the best but my favorite grass starter is Treecko.
Design: Purely awesome! Sceptile looks so cunning and like a stalking predator that can instantly pounce anyone unnoticed.
In game: It has a very satisfying stats and moves. I always enjoy having Sceptile in my team.
It's such a shame that Chikorita becomes Meganium after his 2nd evolution. Chikorita is my favourite starter pokemon and it's one of the cutest pokemon for me. Bayleef is still good and cute. But Meganium... It's not strong enough, it's not good enough, it's not cute entirely.
I posted this a while ago. I still choose Chikorita&Bayleef over Treecko&Grovyle. But I'm watching Hoenn episodes again and when it comes to Meganium vs. Sceptile, Sceptile FTW. Hell yeah. I'm gonna say it. Sceptile is the best fully-evolved grass Pokemon.