What do you want to play as? What style of game? What are the PCs doing? And medieval and early modern is a wide range. For that matter what level of crunch do you enjoy, and what level of authority over the gameworld do you think the GM should have?Real world historical with the addition of some supernatural elements. Particularly medieval or early modern Europe. Is this something Zweihander can do?
I'd look at Fate or Blades in the Dark for many campaigns depending how gritty I wanted it. Warhammer Fantasy Roleplay (or Zweihander) does certain late medieval and early modern well, Pendragon does its setting well. GURPS is excellently researched and gritty, although I'd recommend the sorucebooks more than the system.