If you ask Gilderoy Lockhart, there are few wizards in the world as powerful and accomplished as he is. Don't believe him? Read his book. In it, he reveals all of the exploits and adventures that he set out on and all of the dark and magical creatures he encountered
along the way.
The problem is that the book was all lies and exaggerations. It was revealed in Harry Potter and the Chamber of Secrets that Lockhart mastered a Memory Charm that forced people to forget what really happened so that he could take credit for various events. However, the former Ravenclaw student ended up on the receiving end of the Memory Charm and was later institutionalized.