OK, my top five picks (almost kinda sorta in order)
Germany- being a warmonger myself, I find militaristic useful (gotta love getting more GLs), scientific is always good for the free advances, and in the Industrial Age (the last chance to really redraw the map), the Panzer is the undisputed king.
Vikings- again militaristic, and expansionist is good depending on what level Barbs we're playing. Plus, who can possibly dislike a resourceless, fairly cheap unit available in the mid-Middle Ages (invention) with the attack of a Cavalry with only slightly less defence?
Ottomans- again Scientific, and against thinking human players who understand how to predbuild and use other building strategies, Industrious can be a big advantage. Plus, the Sipahi are useful throughout the entire Late Middle and Industrial Ages until the advent of Infantry. If used properly, the Sipahi can destroy anything in their path until the mid Industrial Age.
China- Yes, I actually prefer China over the Arabs. The Rider (4/3/3) has all the good points of the Ansar, without the lost defense. Also, Militaristic and Industrious combine to make for unstoppable military production, if we choose that path.
Arabs- Expansionist is good when it comes to dealing with high-level Barbs and carving out a nice empire, and Religious is always helpful if we want a cultural win or when we need to change governments. Plus, if we play a big map, Ansars will be a huge asset.