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.If you are looking to diversify your portfolio, consider Hartford Global Growth HLS IA (HIALX). HIALX 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. This fund is a winner, boasting an expense ratio of 0.8%, management fee of 0.75%, and a five-year annualized return track record of 13.47%.MassMutual Select Mid Cap Growth R3 (MEFNX) is a stand out amongst its peers. MEFNX 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 five-year annualized performance of 10.29%, expense ratio of 1.41% and management fee of 0.68%, this diversified fund is an attractive buy with a strong history of performance.Artisan Developing World Advisor (APDYX): 1.15% expense ratio and 1.03% management fee. APDYX is a Non US - Equity fund. Many of these funds like to allocate across emerging and developed markets, and will often focus on all cap levels. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 14.94% over the last 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 (APDYX): Fund Analysis Report Get Your Free (HIALX): Fund Analysis Report Get Your Free (MEFNX): Fund Analysis Report To read this article on Zacks.com click here. Zacks Investment Research