Greninja is easily the best starter this gen. It's got just the right combination of Stats, movepool and type (though that last bit is made redundant) to do what it needs, and it also got a GODLY hidden ability. Getting STAB on every single attack EVER is awesome. Greninja can actually sweep someone if they're not careful, because Greninja's defences, while bad, do ensure it can take at least one hit and deal back a devastating life-orb boost. Unlike most sweepers, though, Greninja SUCKS with a Choice Item, due to it's niche being that it can switch types on the fly.
Sonic's the name, speed's my game!