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 I (DLDRX Quick QuoteDLDRX - Free Report) : 0.89% expense ratio and 0.75% management fee. DLDRX is classified as a Sector - Energy mutual fund. Throughout the massive global energy sector, these funds hold a wide range of quickly changing and vitally important industries. With annual returns of 14.32% over the last five years, this fund is a winner.

Great-West Multi Manager Large Cap Growth (MXLGX Quick QuoteMXLGX - Free Report) : 1% expense ratio and 0.62% management fee. MXLGX is a Large Cap Growth option; these mutual funds purchase stakes in numerous large U.S. companies that are expected to develop and grow at a faster rate than other large-cap stocks. With yearly returns of 14.95% over the last five years, MXLGX is an effectively diversified fund with a long reputation of solidly positive performance.

Virtus KAR Small-Cap Core I (PKSFX Quick QuotePKSFX - Free Report) : 1% expense ratio and 0.75% management fee. PKSFX is a Small Cap Blend mutual fund, and usually targets stocks with market caps of less than $2 billion, letting investors diversify their funds among other kinds of small-cap equities. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 13.52% over the last five years.

We hope that your investment advisor (if you use one) has you invested in one or all of the top-ranked mutual funds we've reviewed. But if that isn't the case, it might be time to have a conversation or reconsider this vitally important relationship.