I had the greatest success and fun with Poland. I went from becoming the richest, to the most feared, to the most hated (because I conquered a lot), and ended up as the global leader. A really messy game but it was really fun for me. Winged hussars are awesome! The ability to push back enemies is really cool, and extra damage dealt if they can't be pushed back anymore... I loved destroying The Zulus with my Hussars. Winged Hussars were both great in real life hundreds of years ago, and in Civ5. Plus, the free social policy per era advancement is really neat. I easily maxed out many policies, which made my nation ahead of others.
I really like the Germans. I always play with them and I think that they give you a huge advantage. I sometimes play as civilizations that are unique to some of the well known and mainstream ones.
I usually have my one or two go to tribes, but I did think that in this version particularly I might have tried a little more variety, which is probably a good thing.
