Americas for sure. I was getting tired of the old "throw spearmen at enemies to hold the lines then smash into their sides with cav until they die" routine, so America is a breath of fresh air. I'm loving my Apaches. They die fairly quickly, but it's so awesome to run everywhere without a worry, and their arches destroy other native factions (and are decent against Spain)

Second favorite is Britannia, because I've always loved that time period, and Scotland is my favorite vanilla faction. Haven't tried Crusades more than a few custom battles, but it seems kinda meh, and played a bit on Teutonic and after I get past the "WOW! LOOKIT THE HELMETS!" stage, it seems just like normal Medieval 2 but with some different models, a useless unit of a tiny number of clergymen, and the annoying religion factor.