Investing in mutual funds for retirement is never too late. And the Zacks Mutual Fund Rank can be an excellent tool for investors looking to invest in the best funds.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 take a look at some of our top-ranked mutual funds with the lowest fees.Oppenheimer Discovery I (ODIIX) has a 0.65% expense ratio and 0.59% management fee. ODIIX is a Small Cap Growth mutual fund and tends to feature small companies in up-and-coming industries and markets. With yearly returns of 13.58% over the last five years, this fund clearly wins.Columbia Dividend Income Fund R4 (CVIRX) is a stand out amongst its peers. CVIRX 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. With five-year annualized performance of 10.59%, expense ratio of 0.65% and management fee of 0.53%, this diversified fund is an attractive buy with a strong history of performance.DFA US Large Cap Growth Institutional (DUSLX) is an attractive large-cap allocation. DUSLX 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. DUSLX has an expense ratio of 0.18%, management fee of 0.15%, and annual returns of 12.99% over the past five years.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 UpWant 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 (CVIRX): Fund Analysis Report Get Your Free (DUSLX): Fund Analysis Report Get Your Free (ODIIX): Fund Analysis Report To read this article on Zacks.com click here. Zacks Investment Research