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 take a look at some of our top-ranked mutual funds with the lowest fees.

Cambiar Opportunity I (CAMWX Quick QuoteCAMWX - Free Report) : 0.65% expense ratio and 0.6% management fee. CAMWX 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 annual returns of 10.87% over the last five years, this fund is a winner.

Dreyfus Strategic Value I (DRGVX Quick QuoteDRGVX - Free Report) : 0.68% expense ratio and 0.6% management fee. DRGVX 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. DRGVX, with annual returns of 11.26% over the last five years, is a well-diversified fund with a long track record of success.

Wells Fargo Large Cap Growth I (STNFX Quick QuoteSTNFX - Free Report) is an attractive large-cap allocation. STNFX 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. STNFX has an expense ratio of 0.84%, management fee of 0.69%, and annual returns of 10.21% over the past five years.

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.