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.

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

Oppenheimer Discovery I (ODIIX Quick QuoteODIIX - Free Report) : 0.67% expense ratio and 0.63% management fee. 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. ODIIX has achieved five-year annual returns of an astounding 11.48%.

MFS Research R2 (MSRRX Quick QuoteMSRRX - Free Report) . Expense ratio: 1.06%. Management fee: 0.43%. MSRRX 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. This fund has managed to produce a robust 10.45% over the last five years.

MSIF Global Quality Portfolio I (MGQIX Quick QuoteMGQIX - Free Report) is an attractive large-cap allocation. MGQIX is a Global - Equity mutual fund, which invests their assets in large markets, leveraging the global economy. MGQIX has an expense ratio of 0.89%, management fee of 0.7%, and annual returns of 10.2% 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.