Serperior, which is, at least relative to other starters (of all generations), fast and defensive which is a rare configuration for anything in any video game. I guess it's a physical Sceptile plus access to Coil, but I'm not very familiar with starters since I don't use them; and Serperior is, despite its novel stat distribution, not built to how I would play a Grass-type though I would consider it for a Grass monotype challenge.
My expressed views don't seem very positive... it's not this generation, I'm just not enthusiastic about starters in general.