It is never too late to invest in mutual funds for retirement. As such, if you plan to invest in some of the best funds, the Zacks Mutual Fund Rank can provide you with valuable guidance.

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 investors' portfolios.

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

Vanguard Equity Income Admiral (VEIRX): 0.17% expense ratio and 0.16% management fee. VEIRX is a Large Cap Value mutual fund, which invests in stocks with a market cap of $10 billion of more, but whose share prices do not reflect their intrinsic value. With annual returns of 11.52% over the last five years, this fund is a winner.

Fidelity Mega Cap Stock (FGRTX Quick QuoteFGRTX - Free Report) : 0.58% expense ratio and 0.57% management fee. FGRTX is a Large Cap Blend fund, targeting companies with market caps of over $10 billion. These funds offer investors a stability, and are perfect for people with a "buy and hold" mindset. FGRTX, with annual returns of 16.68% over the last five years, is a well-diversified fund with a long track record of success.

Voya LargeCap Growth R (VGORX): 1.15% expense ratio and 0.51% management fee. VGORX 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 a five-year annual return of 9.23%, this fund is a well-diversified fund with a long track record of success.

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.