My experiences:
Norse: Best infantry (good armours for low level troops, best archers (only nation with armoured archers), no cavalry at all
Picts: Below average infantry, no archers but high level skirmishers, good cavalry
Irish: Average infantry, no archers but high level skirmishers, best cavalry (only cavalry with chainmail)
Angles/Saxons: Average in every way, good all round army
Frisians: Only 4 troop types, upgrade very fast, easy to build a decent army with them, but no high end troops at all
South Celts (Britons/Welsh): Below average infantry (good weapons but horrible armours), below average ranged troops (slingers), average cavalry