I think the Germans have the best. You can play the entire game with just their spearman. Plus by the time you get halfway through they are very tough troops. The German Cavalry is cheap and good. While some of the more specialized infantry are nice to have around they are slow to build, require special structures, and if you are an advancing army this will make them hard to reinforce. The Axemen are good enough and only slightly less capable than NightRaiders. By the time you get them you should have sufficient leadership and experience to make them work. Remember you have to keep your army moving and take those provinces fast or the enemy will counter-attack. Germany has that ability.
Carthage, (and others) are resource restricted. They just can't build elephant units anywhere they want. This makes conquering Europe a little tedious if you put your elephants into the fray.

Countless times I have smashed armies 5 times my strenght with just German noble cavalry and barbarian cavalry. If I have spearman they can hold the line long enough for my cav to flank and the powerful charge bonus does the rest.