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.Great performance, diversification, and low fees: it's a pretty simple formula for a great mutual fund. Some are better than others, but utilizing our Zacks Rank, we have identified three mutual funds that would make great additions to long-term investors' portfolios.Let's learn about some of Zacks' highest ranked mutual funds with low fees you may want to consider.American Beacon Stephens Mid Cap Growth I (SFMIX) has a 0.89% expense ratio and 0.8% management fee. SFMIX is a Mid Cap Growth mutual fund. These funds aim to target companies with a market capitalization between $2 billion and $10 billion that are also expected to exhibit more extensive growth opportunities for investors than their peers. With yearly returns of 13.42% over the last five years, this fund clearly wins.LKCM Equity Institutional (LKEQX) is a stand out amongst its peers. LKEQX 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 five-year annualized performance of 10.38%, expense ratio of 0.8% and management fee of 0.7%, this diversified fund is an attractive buy with a strong history of performance.T. Rowe Price Japan Fund (PRJPX): 0.97% expense ratio and 0.79% management fee. PRJPX is a Japan - Equity fund option, and these funds primarily invest in companies in one of the world's richest and most diversified economies, Japan. With a five-year annual return of 10.4%, this fund is a well-diversified fund with a long track record of success.So, there you have it - if your advisor has you invested in any of our "Magnificent Retirement Mutual Funds," they are certainly earning their keep. If not, you may want to look elsewhere.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 (LKEQX): Fund Analysis Report Get Your Free (PRJPX): Fund Analysis Report Get Your Free (SFMIX): Fund Analysis Report To read this article on Zacks.com click here. Zacks Investment Research