...Civ III used to have an exploitable bug that gave you double resources for sacrificing population, Civ IV you used to be able to steal workers and chop forests. I don't think I would have been able to beat any of them on the highest difficulty without exploits. I couldn't find any easy exploits in Civ V outside of the AI being atrocious at war...
Thank you for the quick reminder why I was so disappointed with III and IV, and why I think V is so much better. Since II, I just played the game as it was laid out by the developer, not looking for exploits. II could be won straight-up, at the highest difficulty setting, without caravans or other exploits. SMAC as well. That was not true for III and IV, and players really did need to chase exploits. I hated that, and V is much better in this regard. Note that the AIs in III and IV were also atrocious at war, it is just that stacks-of-doom masked the defects.
Coming back to this tier list, science being so much more important than religion or production, it makes perfect sense that Celts and Rome don't make the cut. But do check out consentient's list as he lays out a methodology for numerical ranking. Other tier lists, including this one, end up being much subjective.