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 retirement investors' portfolios.

Here are the funds that have achieved the Zacks Mutual Fund Rank #1 (Strong Buy) and have low fees.

If you are looking to diversify your portfolio, consider

American Beacon Small Cap Value Adviser

(AASSX Quick QuoteAASSX - Free Report) . AASSX is a Small Cap Value mutual fund option, which typically invest in companies with market caps under $2 billion. This fund is a winner, boasting an expense ratio of 1.28%, management fee of 0.7%, and a five-year annualized return track record of 11.22%.

T. Rowe Price Cap Appreciation Adviser

(PACLX Quick QuotePACLX - Free Report) : 0.97% expense ratio and 0.59% management fee. PACLX is an All Cap Value mutual fund, which invests in small, medium, and large-cap companies, though they end up focusing on bigger firms due to percentage of assets. PACLX, with annual returns of 11.13% over the last five years, is a well-diversified fund with a long track record of success.

Invesco Comstock R5

(ACSHX Quick QuoteACSHX - Free Report) : 0.52% expense ratio and 0.38% management fee. ACSHX 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 a five-year annual return of 14.04%, 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.