Check their stats, if their speed is low make sure the defense is high etc. There is no "best" starter because there's no best type, but if you still need advice I chose Tepig and he's treating me just fine.
@Porygon: Probably when you first encounter them in a way like Mespirit or Cresselia in D/P/P. I haven't actually seen how you activate them though. If you talk to them, and then they run off, then thats when the Nature and IVs are decided.
Another question... Is Milstraton (something like that?) the right cave to activate the Cobalion/Terrakion/Virizion trio? (I've beaten the E4 and everything)
Whats a good way to train a bad Pansear? I really like Pansear, but it is Lv 20, and is really bad and always 1shotted. A good place to train? (BTW, at gym 6)