Your most commonly trained unit

[Craterus]

My longest RTW campaign was with Egypt/Scythia, so it might have been Desert Cavalry/Horse Archers.
Gallowglasses on MTW. Or um, the spearmen at the start, I can't remember the name.
STW: Yari Samurai

[Lord Chu]

M2TW : Spearmen
RTW: Hastati

[Caerfanan]

Good question indeed...

I almost only play MTW... I would say it depend on the factions...

Vikings in VI: axes, axes, axes (huscarles, landsmen, joms..)
Scots, Picts, Welsh, Irish: High attack, low defense (gallowglasses, clansmen, warriors)
Mercs, saxons, Northumbria: balanced but seeks the huscarles.

Oter than VI:

east troops: Steppe heavy cav/ Szekely / Boyards
Otherwise I love having archers with a potential melee capacity and/or speed: futtuwa, trebizond archers, bulgarian bandits... and I tend to "over produce" them

[Rhyfelwyr]

STW - Geisha's
STW:MI - never played the campaign more than a couple of turns
MTW - Urban Militia (never got far into campaigns, too impatient at the time)
MTW:VI - Highland Clansmen
RTW - Principes, Chosen Swordsmen
RTW:BI - barely played it
M2TW - Italian Spear Militia (used them tons as both Venice and Sicily)
M2TW:K - still to get