Yes, someone could beat him. He "could" lose to a select number of fighters, but theyd have to get lucky, and fight a great fight. Mosley, Paul Williams, or Margarito would be a difficult fight for him. If he doesnt retire, yes he will remain P4P best until he is beaten. Do you want to know what the most entertaing perspective fight would be, or the most entertaining hes already had?
Id say most entertaining of his that Ive seen was the Zab Judah fight. Lots of drama surrounding the fight, and during the fight. Low blows, cornenrs about to duke it out, and a damn good fight.
Perspective fight, any one of the 3 listed above, but I personally would love to see him fight Margarito or Williams.