The past decade has been a challenge for banks, given the financial crisis and its aftermath, but one upside is that it's given investors the opportunity to see which of the nation's biggest lenders actually operate in a prudent and profitable manner as opposed to just claiming to do so.With respect to the nation's largest banks, three in particular have separated themselves from the pack: Wells Fargo (NYSE: WFC), PNC Financial (NYSE: PNC), and U.S.