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 Advisor Convertible Security Institutional (FICVX Quick QuoteFICVX - Free Report) : 0.74% expense ratio and 0.55% management fee. FICVX is a Convertible Bonds mutual fund, and these funds are unique in the fixed income world; these securities have components of both fixed income and equity, making them hybrid securities. With annual returns of 10.65% over the last five years, this fund is a winner.

MFS Mass Investors Growth Stock R2 (MIRGX Quick QuoteMIRGX - Free Report) : 0.96% expense ratio and 0.33% management fee. MIRGX 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. With yearly returns of 10.98% over the last five years, MIRGX is an effectively diversified fund with a long reputation of solidly positive performance.

State Street Institutional US Equity Services (SUSSX Quick QuoteSUSSX - Free Report) : 0.65% expense ratio and 0.37% management fee. SUSSX 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. With a five-year annual return of 10.37%, this fund is a well-diversified fund with a long track record of success.

There you have it. If your financial advisor had you put your money into any of our top-ranked funds, then they've got you covered. If not, you may need to talk.