I mean black thought is nice in his own right but to be the greatest you would have to do more than just be a great technical MC. Besides I never subscribe to greatest of all time theories because to even come close to being accurate you would have to have a clear beginnig and ending of a Genre or whatever and take a look at who made the greatest impact on that artistic meduim. I mean Thought is no doubt one of the nicest hands down but what has been his subtantial contribution to totally change the direction of the culture called hip-hop.
Here is an example Nobody was rapping like rakim before he came out when he came he brought a style that many artist reference to this day. Same thing with Nas or take the way run dmc dressed and mixed rock and rap in a way nobdies has since(and done it that well). Black thought will always be considered one of the greatest though I mean he has an Iron Clad flow and nice cadence and a voice that's distinctive but he hasn't had his moment to redifine what is rap to the world yet and lets not use the band thing as an example cause that is more Questloves baby per say.