If you're invested in any of the funds in our "Magnificent Retirement Mutual Funds" list, congratulations on owning some of the best managed and top-performing mutual funds. If you are lucky enough to discover our list of Top-Ranked Funds for the first time, it's never too late to start investing with the best, especially when it comes to your retirement.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 our Zacks 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 #1 (Strong Buy) Zacks Rank and have low fees.MSIF Global Quality Portfolio A (MGQAX) has a 1.25% expense ratio and 0.7% management fee. MGQAX 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 yearly returns of 10.19% over the last five years, this fund clearly wins.Fidelity OTC Portfolio (FOCPX): 0.89% expense ratio and 0.73% management fee. FOCPX 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. FOCPX, with annual returns of 18.25% over the last five years, is a well-diversified fund with a long track record of success.Janus Henderson Global Technology I (JATIX). Expense ratio: 0.75%. Management fee: 0.64%. Five year annual return: 23.62%. JATIX is part of the Sector - Tech mutual fund category that invests in technology and lets investors own a stake in a notoriously volatile sector, but with a much more diversified approach.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.Do You Know the Top 9 Retirement Investing Mistakes?Investing in underperforming mutual funds is just one of the key errors that can derail your retirement plans.To learn more, read our just-released report: 9 Retirement Mistakes You Need to Avoid.This report can help you sidestep these costly mistakes and potentially achieve your retirement goals. Get Your FREE Guide Now Get Your Free (FOCPX): Fund Analysis Report Get Your Free (MGQAX): Fund Analysis Report Get Your Free (JATIX): Fund Analysis Report To read this article on Zacks.com click here. Zacks Investment Research