Your most commonly trained unit

[Raz]

STW: n/a
-> MI: n/a

MTW: Camels
-> VI: Any kind of Huscarles

RTW: Camel Cataphracts if i can get them.
-> BI: Haven't really played it enough.
-> Alexander: n/a

MTW2: Most kinds of mounted archers.
-> Kingdoms: n/a

[Ignoramus]

MTW: Urban Militia
RTW: Round Shield Cavalry
MTWII: Spear Militia

[econ21]

I agree with most other posters - it tends to be spearmen. (The only exception is as Rome.) I am not sure if this is because they are good, or instead because they are the worst unit and so die a lot, needing to be replaced. They are cheap, low tech and can hold off the enemy long enough for the real battle winners to do their job.

My killer units tend to be archers and heavy cavalry, but they tend not to die and I keep their numbers down for realism and to maintain the challenge. The poor spearmen form the frontline and so get hammered by everything - archers, cav, swords and enemy spears. Swords tend to be better, but are harder to get and more survivable.

STW: Yari samurai
MTW: Chivalric sergeants (or feudal sergeants on early)
RTW: Hastati or principes
BI: Plumbatarii (so much better than comitatenses)
EB: Hastati or principes
M2TW: Armored sergeants (so much better than spear militia, but still lousy) - mercs if England

[A Norseman]

Hmm.. tricky

MTW: Vikings.
VI: Viking landsmenn
RTW: Hasati or legionaries.
BI: Comitatenses
MTW2: Dismounted feudal knights or Longbowmen.

[Punicus]

RTW: Long Shield Cavalry (Carthage) and Hoplites (Greece)

[Gaius Scribonius Curio]

MTW: Byzantine Infantry

RTW: Horse archers / Persian Cavalry

[Tristuskhan]

STW: not played.

MTW: Saracen infantery and steppe heavy cav (VI version)

RTW, only modded playing, so... RTR; Chalkaspidai and EB; Parthian noble horse archers.

MTW2, not played (are janissaries back?)