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.

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

Clipper Fund (CFIMX Quick QuoteCFIMX - Free Report) has a 0.7% expense ratio and 0.55% management fee. CFIMX is part of the Large Cap Blend section, and these mutual funds most often invest in firms with a market capitalization of $10 billion or more. By investing in bigger companies, these funds offer more stability, and are often well-suited for investors with a "buy and hold" mindset. With yearly returns of 14.76% over the last five years, this fund clearly wins.

American Funds Mutual Fund 529A (CMLAX): 0.6% expense ratio and 0.23% management fee. CMLAX 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 yearly returns of 12.4% over the last five years, CMLAX is an effectively diversified fund with a long reputation of solidly positive performance.

T. Rowe Price Institutional Large Cap Core (TPLGX). Expense ratio: 0.56%. Management fee: 0.55%. Five year annual return: 13.17%. TPLGX 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.

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.