On regent, you should be able to handle any civ, but if you want to spare yourself some headaches, don't take Greece, Rome, or Carthage as opponents. They have the best early defenders.
Also, on the other end of the headache spectrum, is Persia, Ottomans and Iroqouis. They have the best UU attackers. Celts are also a potential on this list as their UU is the same strength but twice as fast.
Finally, other civs that cause headaches not because of UU but because of their danger are Mayan, Sumerian, or almost any agricultural or industrious civ.
You can pick civs that have no UU to speak of, like Portugal. Actually, any seafaring civ on a pangea would be at a disadvantage, some what. England and the Byzantines have ship UUs that are actually decent UUs, but again on a pangea, and especially in the hands of the AI are "mostly harmless."
Are you going for the easiest, and hopefully fastest, best score or are you going for a fun game? These are some guidlines. I have no doubts that you could handle it if you pick four opponents like Maya, Persia, Iroquois and Rome, but do
you want it
hard or
easy?
The game you, idiot!

That's it I quit.
