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.

AQR Small Cap Multi-Style N (QSMNX): 0.88% expense ratio and 0.45% management fee. QSMNX is a Small Cap Blend mutual fund, allowing investors a way to diversify their funds among various types of small-cap stocks. With annual returns of 10.83% over the last five years, this fund is a winner.

Wilshire Large Company Growth Institutional (WLCGX): 1% expense ratio and 0.75% management fee. WLCGX 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 14.6% over the last five years, WLCGX is an effectively diversified fund with a long reputation of solidly positive performance.

Goldman Sachs SmCap Val Insights R6 (GTTUX Quick QuoteGTTUX - Free Report) : 0.83% expense ratio and 0.8% management fee. GTTUX is a Small Cap Value mutual fund option, which typically invest in companies with market caps under $2 billion. With a five-year annual return of 12.07%, 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.