The nation's largest banks have made quite a comeback. Having nearly brought the market to its knees a few months ago, the financial sector has surprised experts with impressive profits. Citigroup -- widely considered to be the country's weakest big bank -- reported this morning net first-quarter income of $1.6 billion, beating analyst expectations and making it the bank's best quarter since 2007, Citi CEO Vikram Pandit said. Citi's news comes on the heels of reports showing better-than-expected, first-quarter profits by three other major banks: $2.1 billion for JPMorgan Chase, $1.7 billion for Goldman Sachs and $3 billion for Wells...