He is in the eu don't know exactly where he comes from. For some unknown reason he is considered the most powerful jedi before yoda. I read about him in some other Forums. He was sad to be able to counter any force attack, as well as defeat any component in saber combat. I have no source, but the five top jedi are only my opinion not fact.
#1 is obviously Luke. Statements from GL and Wiki both put him at double the Emperor's power. The same Emperor who defeated Yoda. From what I can tell, Luke is several teirs above anyone as a Force Weilder, and is within the top tier swordsmasters (Along with Yoda, Kas'im, etc)
Yoda is #2 or #3, depends on whether or not he was superior to the Hero of Tython, who defeated Emperor Vitiate in a Dark Nexus.