There is never a wrong time to invest in mutual funds for retirement. So, if you're still looking for the best mutual funds, the Zacks Mutual Fund Rank can be a great guide.

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 learn about some of Zacks' highest ranked mutual funds with low fees you may want to consider.

Davis New York Venture Fund A (NYVTX Quick QuoteNYVTX - Free Report) : 0.91% expense ratio and 0.54% management fee. NYVTX is a Global - Equity mutual fund investing in bigger markets like the U.S., Europe, and Japan; these kinds of funds aren't limited by geography. NYVTX has achieved five-year annual returns of an astounding 12.98%.

Vanguard Windsor-II Admiral (VWNAX): 0.24% expense ratio and 0.22% management fee. VWNAX is a part of the Large Cap Value category, and invests in equities with a market capitalization of $10 billion or more, but whose share prices do not reflect their intrinsic value. VWNAX, with annual returns of 13.66% over the last five years, is a well-diversified fund with a long track record of success.

Invesco SteelPath MLPSelect 40 R (SPMWX): 1.37% expense ratio and 0.69% management fee. SPMWX 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. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 24.62% 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.