Prudential Financial Investors have largely been ignoring the shares of insurance companies in recent years, in large part because low interest rates are depressing this industry's earnings. Insurers sit on billions of dollars in cash to pay out against future policy claims, and when rates are higher, all that money earns a pretty penny. Prudential ( PRU) is no exception. Shares trade for just 71% of tangible book value. More to the point, this company looks awfully cheap when you look at free cash flow. Prudential is on track for the second straight year of free cash flow in excess of $10 billion, and that figure will likely rise even higher when interest rates start to move up. How do you square such large numbers against this insurer's $27.5 billion market value? Frankly, it's hard to fathom. Analysts at Citigroup think that if you are going to invest in just one insurance stock, you should choose Prudential. According to Citigroup, Prudential "offers the best risk-reward among large cap multi-line life insurers. In our view, the company's improving ROE, above-average near-term earnings growth, and superior business mix merit a valuation above the life group average." To be sure, there isn't much risk in a stock that has already traded so far below tangible book value. Notably, book value is likely to keep on rising in coming years, thanks to the company's impressive free cash flow. Book value per share currently stands at around $79 a share (compared with a stock price of $57). Citigroup's analysts expect book value to approach $85 a share this year and exceed $90 a share in 2014. To see these free cash flow kings in action, visit the 4 Free-Cash-Flow Powerhouses portfolio.