There is never a wrong time to invest in mutual funds for retirement. So, if you're still looking for the best mutual funds, the Zacks Mutual Fund Rank can be a great guide.

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.

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

Columbia Small Cap Value I Class R5 (CUURX Quick QuoteCUURX - Free Report) has a 0.91% expense ratio and 0.81% management fee. CUURX is a Small Cap Value mutual fund, investing in small companies with stock market valuation less than $2 billion. With yearly returns of 11.84% over the last five years, this fund clearly wins.

Fidelity New Millennium (FMILX Quick QuoteFMILX - Free Report) : 0.86% expense ratio and 0.68% management fee. FMILX is an All Cap Growth mutual fund investing in a wide variety of equities, no matter the size of the company and as long as the firm exhibits growth characteristics. FMILX, with annual returns of 15.68% over the last five years, is a well-diversified fund with a long track record of success.

Madison Covered Call & Equity Income Y (MENYX Quick QuoteMENYX - Free Report) . Expense ratio: 1.01%. Management fee: 0.85%. Five year annual return: 11.38%. MENYX 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.

These examples highlight the fact that there are some astonishingly good mutual funds out there. If your advisor has you in the good ones, bravo! If not, you may need to have a talk.