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 learn about some of Zacks' highest ranked mutual funds with low fees you may want to consider.

If you are looking to diversify your portfolio, consider Fiera Capital Sm/Mid-Cap Grwth Inst (APSGX Quick QuoteAPSGX - Free Report) . APSGX is a Mid Cap Growth mutual fund. These mutual funds choose companies with a stock market valuation between $2 billion and $10 billion. This fund is a winner, boasting an expense ratio of 1.05%, management fee of 0.9%, and a five-year annualized return track record of 10.09%.

Fidelity Growth Discovery (FDSVX Quick QuoteFDSVX - Free Report) . Expense ratio: 0.77%. Management fee: 0.6%. FDSVX 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. This fund has managed to produce a robust 12.59% over the last five years.

Arrow Managed Futures Strategy I (MFTNX Quick QuoteMFTNX - Free Report) : 1.29% expense ratio and 0.85% management fee. MFTNX is classified as an Allocation Balanced fund, which seeks to invest in a balance of asset types, like stocks, bonds, and cash, and including precious metals or commodities is not unusual. With a five-year annual return of 13.21%, this fund is a well-diversified fund with a long track record of success.

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.