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 retirement investors' portfolios.

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

If you are looking to diversify your portfolio, consider Buffalo Small Cap Fund (BUFSX Quick QuoteBUFSX - Free Report) . BUFSX is a Small Cap Growth mutual fund and tends to feature small companies in up-and-coming industries and markets. This fund is a winner, boasting an expense ratio of 1%, management fee of 0.85%, and a five-year annualized return track record of 10.15%.

Schwartz Value Fund (RCMFX Quick QuoteRCMFX - Free Report) : 1.25% expense ratio and 0.75% management fee. RCMFX is a Small Cap Value mutual fund option, which typically invest in companies with market caps under $2 billion. With yearly returns of 14.1% over the last five years, RCMFX is an effectively diversified fund with a long reputation of solidly positive performance.

T. Rowe Price Health Sciences (PRHSX Quick QuotePRHSX - Free Report) : 0.8% expense ratio and 0.63% management fee. PRHSX is part of the Sector - Health category, offering investors a focus on the healthcare industry, one of the largest sectors in the American economy. With a five-year annual return of 11.08%, 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.