I'm going to go with Mongols also considering they had one of the largest land empires. Their tactics such as withdraw (bait the enemy) while turning around and firing off salvos of arrows from their horse archers every once in awhile made it almost impossible to track them down and kill them.
I also like the Vikings during the Middle Ages. They weren't incredibly dominate but they were supreme raiders and opportunist. They also settled in the Normandy area (Rollo), helped establish Russia (Rus Vikings), introduced ideas such as all men are created equal, and helped show Europe that they needed to defend their coast lines/ group up together to better defend themselves. They also had the best boats for the time and helped explore a lot. They deserve all the credit for find Americas by boat (not Christopher Columbus).







