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.

The best way to shortlist great mutual funds is to ensure solid performance, diversification, and low fees. Some are better than others, but utilizing the Zacks Mutual Fund Rank, we have identified three mutual funds that could be solid additions to one's retirement portfolio.

Let's take a look at some of our top-ranked mutual funds with the lowest fees.

If you are looking to diversify your portfolio, consider MFS Mass Investors Growth Stock I (MGTIX Quick QuoteMGTIX - Free Report) . MGTIX 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. This fund is a winner, boasting an expense ratio of 0.46%, management fee of 0.33%, and a five-year annualized return track record of 12.54%.

Neuberger Berman Guardian R3 (NGDRX Quick QuoteNGDRX - Free Report) . Expense ratio: 1.36%. Management fee: 0.74%. NGDRX is a Large Cap Value mutual fund, which invests in stocks with a market cap of $10 billion of more, but whose share prices do not reflect their intrinsic value. This fund has managed to produce a robust 11.44% over the last five years.

Oppenheimer Discovery R (ODINX Quick QuoteODINX - Free Report) : 1.26% expense ratio and 0.59% management fee. ODINX is one of many Small Cap Growth mutual funds; these funds tend to create their portfolios around stocks with market capitalization of less than $2 billion. With a five-year annual return of 11.26%, 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.