It really depends..
Swadia has the best cavalry, Nords have the best infantry
Swadia is in probably the worst position on the campaign map though, they're in the middle of it pretty much. Usually gets partitioned in my playthroughs
Vaegirs and Rhodoks have the best ranged units, Rhodok xbow spam is stupidly OP. Don't bother playing Rhodoks in the field though, they're only good at sieges