They all have line infantry. You use line infantry. When in doubt, recruit line infantry.

On a more serious side, there's very few unique units this time around. Some factions don't even have any. Most of the "unique" units are actually the same, just renamed for every faction that has them. I believe France and only 2 other nations have missile cavalry.

Then again, the stats of line infantry does vary between factions. I think the most unique is Austria's. Theirs is significantly weaker but have 150 men on default, instead of 120.