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!

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 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 AB Discovery Growth K . CHCKX 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. This fund is a winner, boasting an expense ratio of 1.08%, management fee of 0.61%, and a five-year annualized return track record of 12.29%.

T. Rowe Price Institutional Large Cap Growth (TRLGX Quick QuoteTRLGX - Free Report) is a stand out amongst its peers. TRLGX 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 five-year annualized performance of 16.85%, expense ratio of 0.56% and management fee of 0.55%, this diversified fund is an attractive buy with a strong history of performance.

Brown Advisory Flexible Equity Institutional (BAFFX Quick QuoteBAFFX - Free Report) : 0.56% expense ratio and 0.44% management fee. BAFFX is classified as an Allocation Balanced fund, which seeks to invest in a balance of asset types, like stocks, bonds, and cash, and including precious metals or commodities is not unusual. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 11.22% 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.

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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.