Land unit is line infantry. Plain and simple, it wins your battles and you can't fight without them.

On water, 2nd rates. They're bloody evil. A friend and i fought a naval battle online where we tested them out. I had a second rate surrounded by 4 4th rates (no experience on either side), he was ripping into me with grape shot, roundshot, every shot you can think of, he barely scratched the 2nd rate, i sank 2, disabled one and one got away.