Hard question! I guess it depends of which type of victory im pursuing. Personally always been quite fond of Immortals and since been playing Civ5 i'll go with dutch's Sea Beggar or Incan's Slinger can be quite funny when wiping annoying european units. My best game was an alliance between Netherlands, Inca, Aztecs and Mayans. They helped me dispatch Spain and England pretty easy, Jaguars and Slingers did most of the job as I pillaged reinforcements arriving on the seas.