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 retirement investors' portfolios.

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

PGIM Jennison Mid-Cap Growth R6 (PJGQX Quick QuotePJGQX - Free Report) has a 0.61% expense ratio and 0.57% management fee. PJGQX is a Mid Cap Growth mutual fund. These funds aim to target companies with a market capitalization between $2 billion and $10 billion that are also expected to exhibit more extensive growth opportunities for investors than their peers. With yearly returns of 11.82% over the last five years, this fund clearly wins.

TIAA-CREF Social Choice Equity Retirement (TRSCX Quick QuoteTRSCX - Free Report) : 0.41% expense ratio and 0.15% management fee. TRSCX is classified as a Large Cap Blend fund. More often than not, Large Cap Blend mutual funds invest in companies with a market cap of over $10 billion. Buying stakes in bigger companies offer these funds more stability, and are well-suited for investors with a "buy and hold" mindset. TRSCX, with annual returns of 10.94% over the last five years, is a well-diversified fund with a long track record of success.

Nuveen Winslow Large-Cap Growth A (NWCAX Quick QuoteNWCAX - Free Report) : 0.91% expense ratio and 0.64% management fee. NWCAX is a Large Cap Growth mutual fund, and these funds invest in many large U.S. firms that are projected to grow at a faster rate than their large-cap peers. With a five-year annual return of 12.82%, 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.