WIN THE DEBATE, PART XVI. When someone says that this or that is the "most" or "best" or "greatest" or "weakest," ask them this question: What was the most or best or greatest before?
They'll rarely have an answer because we typically use superlatives (e.g. most, best, etc) for emphasis, not as fact. When you ask someone what their latest "best" replaces and they can't answer, you reveal that your opponent is offering not fact but opinion. This undercuts their authority and gives you an opening to return to fact.
(If they have a fact-based answer, fine. All you've done is make them back it up. That helps you, too, by keeping the argument in the realm of fact.)