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.

Invesco Small Cap Value Y (VSMIX) has a 0.8% expense ratio and 0.63% management fee. VSMIX is a Small Cap Value mutual fund option, which typically invest in companies with market caps under $2 billion. With yearly returns of 19.31% over the last five years, this fund clearly wins.

Fidelity Infrastructure (FNSTX): 0.94% expense ratio and 0.81% management fee. FNSTX is a Large Cap Blend fund, targeting companies with market caps of over $10 billion. These funds offer investors a stability, and are perfect for people with a "buy and hold" mindset. FNSTX, with annual returns of 11.4% over the last five years, is a well-diversified fund with a long track record of success.

MFS Blended Research Growth Eq C (BRWCX Quick QuoteBRWCX - Free Report) . Expense ratio: 1.45%. Management fee: 0.25%. Five year annual return: 12.26%. BRWCX 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.