Shares of the largest US banks proved to be among the best investments in 2012. The question many investors now face is whether they can pull off an encore.

The financial sector of the Standard & Poor's 500-stock index has soared 25% in 2012, outpacing the broader S&P 500 index, which is up 12%. The KBW Bank Index, which tracks 24 of the country's biggest banks, has gained 28%.