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 SMID Fund Investor (CAMMX Quick QuoteCAMMX - Free Report) : 0.92% expense ratio and 0.8% management fee. CAMMX is a Mid Cap Value mutual funds that aims to target medium-sized companies that possess strong value and income opportunities for investors. CAMMX has achieved five-year annual returns of an astounding 9.21%.

Loomis Sayles Growth Fund A (LGRRX Quick QuoteLGRRX - Free Report) : 0.92% expense ratio and 0.5% management fee. LGRRX is a Large Cap Growth option; these mutual funds purchase stakes in numerous large U.S. companies that are expected to develop and grow at a faster rate than other large-cap stocks. With yearly returns of 13.02% over the last five years, LGRRX is an effectively diversified fund with a long reputation of solidly positive performance.

American Funds Investor Company of America R5 (RICFX Quick QuoteRICFX - Free Report) . Expense ratio: 0.32%. Management fee: 0.23%. Five year annual return: 9.75%. RICFX 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.