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.

The easiest, most reliable way to judge a mutual fund's quality over time is by analyzing its performance, diversification, and fees. The Zacks Mutual Fund Rank, which covers over 19,000 mutual funds, has helped us identify three outstanding options that are perfect for any long-term investors' portfolios that is retirement-focused.

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

If you are looking to diversify your portfolio, consider American Century Small Cap Growth A (ANOAX Quick QuoteANOAX - Free Report) . ANOAX 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 1.42%, management fee of 1.17%, and a five-year annualized return track record of 10.42%.

Columbia Small Cap Value I Class R4 (CVVRX Quick QuoteCVVRX - Free Report) is a stand out amongst its peers. CVVRX is a Small Cap Value mutual fund option, which typically invest in companies with market caps under $2 billion. With five-year annualized performance of 9.69%, expense ratio of 1.01% and management fee of 0.81%, this diversified fund is an attractive buy with a strong history of performance.

AQR Small Cap Multi-Style I (QSMLX Quick QuoteQSMLX - Free Report) : 0.6% expense ratio and 0.45% management fee. QSMLX is a Small Cap Blend mutual fund that usually targets companies with a market capitalization of less than $2 billion. With a five-year annual return of 10.04%, 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.