There is no set criteria. However, you will find that most widely accepted 'great' fighters have excellent resumes. Opposition is a crucial ingredient, however it isn't the be all and end all. Take for example - Bob Foster. He doesn't have the greatest resume you'll ever see, but he is almost universally seen as a top 10, sometimes top 5 light heavyweight. I guess its a combination of what you said, how they look on film... etc.
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