With Civ5 you probably want to use Vox Populi. It really improves the game to a completely new level, including the AI. It's not a human opponent but much better than any Civ version ever shipped officially.

Vox Populi is a DLL mod though so Windows only. This mod is basically the only reason I keep Windows around.

Edit: and forgot to say that the development and community is very active, 12 years after the game originally shipped. New versions come out regularly.