I'll go over all the starters, let you make your own decision as to what you feel is the best.
Snivy-Servine-Serperior: This one is probably the worst of the starters for ingame uses. Currently it is the best of the Gen V starters for competitive use, but that is because of its powerful Hidden Ability. Without access to that, it is easily the weakest of the starters. Its stats are alright, although offensively it is not very powerful. Its speed is fantastic though. The biggest problem with Serperior is that it has the usual Grass type problem of having a very small movepool. Serperior has no coverage at all, so it ends up being a detriment to your team often. It's also outclassed by other Grass types. Lilligant, Whimsicott, Sawsbuck, Ferrothorn, Amoonguss, and Virizion are all better Pokemon.
Tepig-Pignite-Emboar: This is comparatively the strongest of the starters. Fire/Fighting is a good offensive typing, it has good bulk, a good movepool, and its Attack is quite high. It is held back by a pitiful Speed stat, but that is a minor issue for ingame. The biggest problem with Emboar is that it faces the stiffest competition from other Fire and Fighting types. The quality of Fire types is incredibly high, with powerhouses in Darmanitan and Chandelure. The Fighting types are also very powerful, with Conkeldurr, Scrafty, Mienshao, and the Legendary Sword Trio all being better, and Sawk being about on par with the powerlevel of Emboar.
Oshawott-Dewott-Samurott: Mediocre stats all around, but it has a solid movepool and doesn't face as much competition from others of its type. Seismitoad and Jellicent are both better Pokemon than it. The usefulness of a Water type Pokemon on your team early on giving you access to Surf is nice, and it learns Razor Shell at an early level, which is pretty strong. I would pick this as my starter, although I wouldn't expect much out of it thanks to pretty bland stats.