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.

Invesco Value Opportunities A (VVOAX): 0.99% expense ratio and 0.6% management fee. VVOAX 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. VVOAX has achieved five-year annual returns of an astounding 17.47%.

Kopernik International I (KGIIX Quick QuoteKGIIX - Free Report) : 1.01% expense ratio and 0.9% management fee. KGIIX is a Non US - Equity fund. Many of these funds like to allocate across emerging and developed markets, and will often focus on all cap levels. With yearly returns of 9.49% over the last five years, KGIIX is an effectively diversified fund with a long reputation of solidly positive performance.

Vanguard Capital Opportunity Admiral (VHCAX): 0.35% expense ratio and 0.31% management fee. VHCAX is a part of the Large Cap Growth mutual fund category, which invest in many large U.S. companies that are expected to grow much faster compared to other large-cap stocks. With a five-year annual return of 14.53%, 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.