I guess I'd pick generals bodyguards - cavalry is very powerful in M2TW. The only downside is its fragility, but the 2HP/regenerating properties of bodyguards overcomes this.

Of the regular troops, playing England (or HRE) I fixate on armoured swordsmen (or DFK). They are very valuable in dealing with the mass of spearmen the AI often fields and are almost required in sieges to cope with their AI equivalents. Behind stakes, backed by longbows, they can take on most comers.