Luke didn't do anything. All he did was go and confront Palpatine and Vader because he knew there was still good in Anakin. Anakin was the one who actually did anything important. All luke did was sit there and get fried by Palps lightning.
In knowledge force and general Jedi-ness Yoda is the obvious choice with Obi-Wan coming a close second. Luke was a Padawan with (in terms of old republic Jedi) hardly any training time at all.
I see no reason for Luke to be "the greatest Jedi ever" to be perfectly honest. The only thing he's got is that he saw a glimmer of hope in Anakin.
Luke is described by Yoda as the fastest learner ever, so no doubt he is a great Jedi. His trial was to confront Vader and he did. He did what you call the greatest Jedi ever failed to do. Cause being a Jedi is more than just fighting. Luke understood what was wrong with Vader while Obi-Wan failed to miss it. Obi-Wan was too blind by his own emotions, what almost happened to Luke when Vader tempted Luke by treatening his sister. Obi-Wan said that Sith deal in absolutes, so did he! Face it...The old Jedi order wasn't perfect, with rules that drew not only the Chosen one mad but many other Jedi. Even Obi-Wan admits that Anakin's downfall was his own fault.