American Tower Corporation ( AMT 0.59% ) may be the strongest performing stock Wall Street doesn't know about. Over the last 14 years, the leasor of wireless and broadcast communications infrastructure has achieved annualized total returns of 16.3% versus the S&P 500's return of 6.8%. In the last year, shares have done even better, gaining 20% versus the S&P's 6% rise. Savvy investors have quietly trounced the market by buying its stock.