Investing in mutual funds for retirement is never too late. And the Zacks Mutual Fund Rank can be an excellent tool for investors looking to invest in the best funds.

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 break down some of the mutual funds with the top Zacks Mutual Fund Rank and the lowest fees.

PGIM QuantSolutions SmCap Value Z (TASVX): 0.8% expense ratio and 0.6% management fee. TASVX is a Small Cap Value fund, and these funds are known for investing in companies with market caps under $2 billion. TASVX has achieved five-year annual returns of an astounding 10.11%.

Impax US Sustainable Economy A (PXGAX Quick QuotePXGAX - Free Report) is a stand out amongst its peers. PXGAX is a Global - Equity mutual fund, which invests their assets in large markets, leveraging the global economy. With five-year annualized performance of 12.6%, expense ratio of 0.7% and management fee of 0.62%, this diversified fund is an attractive buy with a strong history of performance.

American Funds Growth Portfolio R6 (RGWGX): 0.01% expense ratio and 0% management fee. RGWGX 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 10.31%, 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.