Investing in mutual funds for retirement is never too late. And the Zacks Mutual Fund Rank can be an excellent tool for investors looking to invest in the best funds.

How can you tell a good mutual fund from a bad one? It's pretty basic: if the fund is diversified, has low fees, and shows strong performance, it's a keeper. Of course, there's a wide range, but using the Zacks Mutual Fund Rank, we've found three mutual funds that would be great additions to any long-term retirement investors' portfolios.

Let's take a look at some of our top-ranked mutual funds with the lowest fees.

BNY Mellon Natural Resources A (DNLAX Quick QuoteDNLAX - Free Report) has a 1.14% expense ratio and 0.75% management fee. DNLAX is a Sector - Energy mutual fund, which encompasses a wide range of vastly changing and vitally important industries throughout this massive global sector. With yearly returns of 9.67% over the last five years, this fund clearly wins.

Fidelity Select Health Care (FSPHX Quick QuoteFSPHX - Free Report) : 0.68% expense ratio and 0.53% management fee. FSPHX is a Sector - Health mutual fund, which give investors an opportunity to focus on healthcare, one of the largest sectors of the American economy. FSPHX, with annual returns of 8.83% over the last five years, is a well-diversified fund with a long track record of success.

Oppenheimer Gold & Special Mineral R (OGMNX Quick QuoteOGMNX - Free Report) is an attractive large-cap allocation. OGMNX is classified as a Sector - Precious Metal fund, and these mutual funds invest in stocks with a focus on the mining and production of precious metals like gold, silver, platinum, and palladium. OGMNX has an expense ratio of 1.32%, management fee of 0.58%, and annual returns of 9.13% over the past five years.

These examples highlight the fact that there are some astonishingly good mutual funds out there. If your advisor has you in the good ones, bravo! If not, you may need to have a talk.