Gothic Knights if you start in the Early period ... You'll probably finish the game before they become available since you'll need to play as the German or Italians (Never seen the IA produce them). Kept a few games going despite everything indicating that I could not loose but the only challenge was to find them a worthy opponent ... Not much fun using them just to slaughter vanilla spearmen ...

About the same for all high-level units (Janissaries, Gendarme, etc) once you start in the Early period...