I'm afraid I'm being controversial, but with so many variables in poker, considering a player, a great player, is quite complex.
You don't always get the right reading, you don't always choose the best plays, and if you base your answer on the result, then the answer becomes much more empty. Because in my opinion, there are good players at all levels of money, and the reason they're not playing with supposed great players is simply because they don't have the money to play at a certain level.
Can I make a questionable play in a one-dollar tournament? I can.
Can I make a questionable play in a 50,000 tournament?
I can't. But others can, does that make them great players?
I don't know, but for many, these values are what define whether someone is a great player or not.