Not in any specific order:

A) Szekely-ultra fast horse archer, available VERY early, and dominates a lot of early era infantry straight on.

B) Avar Nobles-Equivalant of Khwarzmian cav. except out of the desert, much smaller requirements, and disciplined.

C) Swiss Halberdiers-Trainable by lots of people, very versatile, can deal with most units.

D) JHI-Really, do I have to say? These guys are monstrous, they slaughter nearly everything, plus have speed, discipline, and the ability to fight in desert.

E) Janissary Infantry- Equivalant to CMAA in hand to hand, except with less armor and with discipline. Plus they have the bow to make them extremely versatile.

F) Armenian Heavy Cavalry- Available very early on, AHC out of Armenia can dominate the battlefield for a very long time.

G) Gothic Knights- Its a pity these guys take so long to get to 'cause they are great. Even with just a strong charge, nearly everything just seems to bounce off them. Recently did a test run of V1 GK w/ +3 armor vs. V1 Lancers w/ +3 armor on a flat European landscape. The lancers went into wedge for the charge and made absolutely NO headway against the GK, and summarily got slaughtered.