I think UU are one of the most interesting and fun parts of the game and pushes the player (in a friendly way) to play a little different with each civ. I think the top 3-5 UU have already been listed. A personal favorite is the Viking Longship, that enables early pillage raids before warmonger costs goes up and of course the ability to steal workers and pick up some extra goodie huts. I also like Alexander's Hetarois. A couple of these, together with a Great General, can cause quite some damage .