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.Let's break down some of the mutual funds with the top Zacks Mutual Fund Rank and the lowest fees.If you are looking to diversify your portfolio, consider Fidelity Growth Discovery K (FGDKX). FGDKX 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. This fund is a winner, boasting an expense ratio of 0.68%, management fee of 0.6%, and a five-year annualized return track record of 12.84%.Arrow Managed Futures Strategy I (MFTNX) is a stand out amongst its peers. MFTNX is a part of the Allocation Balanced fund category; these funds like to invest in a variety of asset types, finding a balance between stocks, bonds, cash, and sometimes even precious metals and commodities; they are mostly categorized by their respective asset allocation. With five-year annualized performance of 10.44%, expense ratio of 1.27% and management fee of 0.85%, this diversified fund is an attractive buy with a strong history of performance.Arrow Managed Futures Strategy I (MFTNX). Expense ratio: 1.27%. Management fee: 0.85%. Five year annual return: 10.44%. MFTNX is a part of the Allocation Balanced fund category; these funds like to invest in a variety of asset types, finding a balance between stocks, bonds, cash, and sometimes even precious metals and commodities; they are mostly categorized by their respective asset allocation.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. Zacks Names "Single Best Pick to Double" From thousands of stocks, 5 Zacks experts each have chosen their favorite to skyrocket +100% or more in months to come. From those 5, Director of Research Sheraz Mian hand-picks one to have the most explosive upside of all. It’s a little-known chemical company that’s up 65% over last year, yet still dirt cheap. With unrelenting demand, soaring 2022 earnings estimates, and $1.5 billion for repurchasing shares, retail investors could jump in at any time. This company could rival or surpass other recent Zacks’ Stocks Set to Double like Boston Beer Company which shot up +143.0% in little more than 9 months and NVIDIA which boomed +175.9% in one year.Free: See Our Top Stock and 4 Runners Up >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Get Your Free (FGDKX): Fund Analysis Report Get Your Free (MFTNX): Fund Analysis ReportTo read this article on Zacks.com click here.Zacks Investment Research