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.

Let's take a look at some of the highest Zacks Ranked mutual funds with the lowest fees.

BlackRock Mid Cap Growth Equity R (BMRRX Quick QuoteBMRRX - Free Report) : 1.3% expense ratio and 0.67% management fee. BMRRX is a Mid Cap Growth mutual fund. These mutual funds choose companies with a stock market valuation between $2 billion and $10 billion. BMRRX has achieved five-year annual returns of an astounding 14.5%.

American Funds Growth Fund of America 529F (CGFFX Quick QuoteCGFFX - Free Report) is a stand out amongst its peers. CGFFX is a Large Cap Growth mutual fund, and these funds invest in many large U.S. firms that are projected to grow at a faster rate than their large-cap peers. With five-year annualized performance of 13.01%, expense ratio of 0.45% and management fee of 0.27%, this diversified fund is an attractive buy with a strong history of performance.

AB Small Cap Growth Adviser (QUAYX Quick QuoteQUAYX - Free Report) . Expense ratio: 0.9%. Management fee: 0.75%. Five year annual return: 14.09%. QUAYX is a Small Cap Growth mutual fund and tends to feature small companies in up-and-coming industries and markets.

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.