I prefer Civ5.
As for Civ4, Huyana Capac beats everyone for Science and Culture victories. Pure math, really. Other strong Financial civs do well too, like Elizabeth or Mansa Musa.
As for Military victories, it's a bit more varied. Every Civ with a strong early-rush unit does well, like Persia or the Mongols. Rome is good as well, because of the Praetorian. I'm not sure who is the best overall, because I usually get too bored to play optimally in the latter parts of Military games; just too much stuff you have to do, even when the winner is clear.