The funds in our "Magnificent Retirement Mutual Funds" list are among the best managed and best performing mutual funds available. If you are just finding out about our Top-Ranked Funds list, we welcome you!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.Let's break down some of the mutual funds with the highest Zacks Rank and the lowest fees.If you are looking to diversify your portfolio, consider Oppenheimer Discovery I (ODIIX). ODIIX is one of many Small Cap Growth mutual funds; these funds tend to create their portfolios around stocks with market capitalization of less than $2 billion. This fund is a winner, boasting an expense ratio of 0.67%, management fee of 0.63%, and a five-year annualized return track record of 11.74%.City Natural Rochdale US Core Equity & Income Service Class (CNRVX): 0.77% expense ratio and 0.4% management fee. CNRVX 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. With yearly returns of 12.27% over the last five years, CNRVX is an effectively diversified fund with a long reputation of solidly positive performance.BlackRock Advantage Small Cap Core I (BDSIX): 0.5% expense ratio and 0.45% management fee. BDSIX is a Small Cap Blend mutual fund, allowing investors a way to diversify their funds among various types of small-cap stocks. With a five-year annual return of 10.1%, this fund is a well-diversified fund with a long track record of success.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 (BDSIX): Fund Analysis Report Get Your Free (CNRVX): Fund Analysis Report Get Your Free (ODIIX): Fund Analysis Report To read this article on Zacks.com click here. Zacks Investment Research