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 investors' portfolios.

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

Recurrent MLP & Infrastructure I (RMLPX): 1.08% expense ratio and 0.9% management fee. RMLPX is a Sector - Energy mutual fund, which encompasses a wide range of vastly changing and vitally important industries throughout this massive global sector. RMLPX has achieved five-year annual returns of an astounding 27.56%.

American Funds Growth Portfolio F1 (GWPFX Quick QuoteGWPFX - Free Report) : 0.38% expense ratio and 0% management fee. GWPFX 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 9.16% over the last five years, GWPFX is an effectively diversified fund with a long reputation of solidly positive performance.

Thornburg Global Opportunities R5 (THOFX): 0.99% expense ratio and 0.83% management fee. THOFX is a Global - Equity mutual fund. These funds invest in large markets like the U.S., Europe, and Japan, and operate with very few geographical limitations The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 14.04% over the last five years.

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.