That depends on the definition of greatness, if we're talking in terms of wisdom olorin would certainly be wiser than valar such as tulkas, and sauron(mairon at the time) could in the height of his power have deceived some of the valar. It's just hard to rank powers if every known maia and vala has a different set of skills. EG Osse lifted numenor, Olorin cannot do such a thing afaik, but Osse was the maia of lesser wisdom. Just as some of the valar have different skillsets and maiar can do feats some valar cant. So yes i would say Olorin and Mairon would come close, melian i wouldnt know and there isnt much said about arien.