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.

Here are the funds that have achieved the Zacks Mutual Fund Rank #1 (Strong Buy) and have low fees.

Fidelity Trend Fund (FTRNX Quick QuoteFTRNX - Free Report) : 0.55% expense ratio and 0.41% management fee. FTRNX 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. With annual returns of 15.44% over the last five years, this fund is a winner.

Goldman Sachs Flex Cap Growth I (GILLX Quick QuoteGILLX - Free Report) : 0.59% expense ratio and 0.55% management fee. GILLX is a part of the Large Cap Growth mutual fund category, which invest in many large U.S. companies that are expected to grow much faster compared to other large-cap stocks. GILLX, with annual returns of 13.35% over the last five years, is a well-diversified fund with a long track record of success.

Matthews Asia Small Companies (MSMLX Quick QuoteMSMLX - Free Report) : 1.32% expense ratio and 1% management fee. MSMLX is a Pacific Rim - Equity mutual fund; these funds typically invest in companies throughout the dominant export-focused markets of Hong Kong, Singapore, Taiwan, and Korea. With a five-year annual return of 14.07%, this fund is a well-diversified fund with a long track record of success.

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.