Not your favorite new unit but your favorite new concept that ES has put into the game. For example, some of the ones that I am fond of are:
-Norse having to kill to gain favor
-16-player support
-the addition of Starcraft-like specialty units (the myth units) in general
-the move to a 3-civ org in which each civ is very different (again like Starcraft and unlike AoE or AoK)
-many of the UI changes
-and the potential (maybe) for ships to be generic and extensible.
But I have to say that my favorite is the fact that the Norse infantry builds their buildings. In some ways this is combined with their kill-to-gain favor, but I think it will just add a tremendous element to the game to be able to be entirely offensive. Gone are the days of having to decide between keeping your villi count up at the pop limit or go for the throat with army at the expense of infrastructure! It's a subtle change yet very novel in my opinion and should really change the way the game is played. Which isn't to say I will necessarily be a Norse player, either, in Starcraft I always played random and actually most enjoyed the Greek-like yet unpopular Terran, and my defensive Age playing style has always fit the Egyptians' strengths. But I think the Norse concepts might be just what the doctor ordered to bring out the nasty in my game!
H, C, C, H, C, C, H, C, C, H, C, C