I agree with @laureth, this is essentially trying to differentiate between fact and opinion. Facts are black and white – true or false (although they are not really facts if they are false!) – while opinions have a vast gradient of gray area.
So how do you determine if something is a fact or an opinion? Good question. It’s often straight-forward, but I’m not sure if you can always definitively say that something is certainly a fact and not an opinion. In fact, I’m not sure if whether this is always possible is a fact or an opinion. :)
One sure-fire way to make a determination is if you can make a measurement, again as @laureth already said. “Best” cannot be measured because there are no set criteria that determine the “goodness” of something. So you could say that pogo sticks of brand #2 are the “best,” but that’s just an opinion because people might disagree on what qualities are considered “good” in a pogo stick. However, if you say that brand #2 pogo sticks produce higher jump heights on average than brand #1 sticks, and the numbers back up your claim, then that is a fact. Whether or not you consider high jump heights a good quality helps to form your opinion on which brand is “better.” In this way, “America is a force for good” is absolutely an opinion because what is “good” to one person or culture isn’t necessarily considered good in another. That is why there are also some people out there who consider America to be “the great Satan” (also an opinion).
But that’s a clear cut example and it gets fuzzy sometimes. For instance, I could say that the New York Yankees are better at baseball than my brother’s little league team. That smacks of opinion (the use of the word “better”) but because we have a set of measureable standards of what makes a baseball team “good at baseball” (high ratio of wins to losses, high ratio of hits to swings, high ratio of homeruns to swings, high pitching speed, etc.) and the Yankees obviously far surpass my brother’s little league team in all those areas, you could almost call this one a fact. In fact, I’m not sure whether its status as fact or opinion is a fact or an opinion, and I’m not sure whether the possibility of making that determination is a fact or an opinion! My brain is starting to hurt.
Then there are things that are certainly facts, but we just don’t know the “answer” yet. Such as your example of free will – since we either do or don’t have free will, there’s no in between, that is a fact and not an opinion (except maybe not because quantum physics is weird). It’s simply a fact we don’t know yet, and so, irritatingly enough, it often gets treated as opinion in debates (e.g. I think we have free will). And in religion, “there exists a god” is either a fact or an untrue statement, and since we don’t know for sure either way, we often treat it as an opinion (e.g. I believe in a god) even though a conjecture about the truth of a statement like that is more appropriately known as a theory or hypothesis (depending largely on the context), not an opinion. And then there are the people who believe that whether or not there is a god is completely unknowable, which further complicates things.
It’s a good question. I hope someone can come in and give you a less wishy-washy answer than mine.