It is not unusual to hear or read statements like, "Alexander the Great was an excellent tactian and strategist" or "the Magna Carta was formative" or "the Thirty Years War was devastating." These types of propositions are routine in history, and are no problem, if you can support them by citing specific facts: "Alexander the Great was an excellent tactician and strategist because he managed to invade and conquer Greece, Persia, and Egypt" or "the Magna Carta was formative because it has shaped not only the government of England, but also the governments of several other nations (USA, Canada, Australia), and has exerted this influence continually over the last 800 years" or "the Thirty Years War was devastating because more people died in it than in any other war prior to 1914." General statements are fine, if they are supported by specific facts.
What are, however, much more problematic, are superlative generalizations. To say that "Alexander the Great was the most excellent tactician and strategist" is very hard to prove: what about Napoleon or Ulyses Grant? To write that "the Magna Carta was the most formative political document every written" is difficult to support: what about the Ten Commandments, or the Declaration of Independence?
So be very careful when using superlatives: "the most" or "the greatest" can get you into trouble. Much safer are "one of the greatest" or "one of the most"; but they still need to be supported by citing details.
