Our "Magnificent Retirement Mutual Funds" list includes some of the best managed and best performing funds around. If you're already invested in these, congratulations! But if you're just now discovering them, don't worry. When it comes to your retirement, it's never too late to start investing in the best.The easiest way to judge a mutual fund's quality over time is by analyzing its performance, diversification, and fees. Using our Zacks Rank of over 19,000 mutual funds, we've identified three outstanding mutual funds that are ideally suited to help long-term investors pursue and achieve their retirement investing goals.Let's learn about some of Zacks' highest ranked mutual funds with low fees you may want to consider.If you are looking to diversify your portfolio, consider Principal Mid Cap Growth R3 (PFPPX). PFPPX is a Mid Cap Growth mutual fund. Mid Cap Growth funds pick stocks--usually companies with a market cap between $2 billion and $10 billion--that demonstrate extensive growth opportunities for investors compared to their peers. This fund is a winner, boasting an expense ratio of 1.24%, management fee of 0.65%, and a five-year annualized return track record of 10.13%.AB Large Cap Growth K (ALCKX): 0.95% expense ratio and 0.51% management fee. ALCKX 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. ALCKX, with annual returns of 15.92% over the last five years, is a well-diversified fund with a long track record of success.T. Rowe Price Japan Fund (PRJPX) is an attractive large-cap allocation. PRJPX is part of the Japan - Equity category, which mostly invest in Japan-based companies; Japan is one of the world's richest and most diversified economies. PRJPX has an expense ratio of 0.97%, management fee of 0.79%, and annual returns of 10.4% over the past five years.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 (ALCKX): Fund Analysis Report Get Your Free (PRJPX): Fund Analysis Report Get Your Free (PFPPX): Fund Analysis Report To read this article on Zacks.com click here. Zacks Investment Research