The third-best performing fund this year, with a 16.5% return through March 22, and a scorching 67% return over the past 12 months -- which makes it the top performer out of all funds in that period -- is the $225 million Integrity Williston Basin/Mid-America Stock Fund ( ICPAX). The fund, which earned a five-star rating from Morningstar, is a niche play, as it focuses on companies active in developing the oil shale and oil sands deposits in a swath of land that runs from the upper Midwest, through North Dakota and into southern Canada. The Williston Basin is one of the largest sedimentary oil shale basins in North America at nearly 300,000 square miles. Integrity Williston Basin/Mid-America also gets a strong four-star rating from Standard & Poor's. Todd Rosenbluth, the ratings firm's mutual fund analyst, said the fund has an excellent long-term performance record, but added that it's worth noting that the three current managers, who work for Integrity Funds, have only been on the job since February 2010. "New management could mean changes to style or strategy and could also make past performance less relevant" in measuring future performance, he cautioned. The fund's top stock pick is National Oilwell Varco ( NOV), at 4.3% of the portfolio. The company is one of the largest equipment suppliers in the drilling industry and has essentially cornered the market for offshore oil rigs. It provides a comprehensive line of equipment for rigs and the products used in oil and gas production. It also provides distribution services, which include maintenance, spare parts and repair services for its equipment. National Oilwell Varco's shares are up 20% this year through March 22 while its industry sector, oil and gas equipment and services, is up 17%, according to Morningstar.