I think that Berserks and Cossacks are some of the best units in the game for their time application. Both of these units allow you to access traits that can't be accessed by any other unit until more modern times, and that beats almost any defensive or offensive advantage. The Berserk, in particular, is absolutely wonderful. Not only does it have 6 attack, unheard of when most civs have musketmen and pikes, but it also is amphibious, meaning that as long s you're attacking a coastal town, it effectively has a movement of 4 or 5.
Musketeers have to be up on top there, too. They're so powerful on the defense, even Cavs have trouble with a couple stacked Musketeers. For their tech, lots more powerful than Swiss mercencaries, comparatively speaking.