I never go for the best. However, I often choose Mudkip, to take along with me throughout my adventures. A perfect match would be a Modest nature so spc. atk. rises. Considering grass(using ice attacks), fire, electric, rock, etc... types are no match. :)
Treecko and Torchic are also unique starters. I give them as much effort as I train any other Pokemon. I'm not too picky in this decision, but I vote for Mudkip, spoiled my answer. xD