The funds in our "Magnificent Retirement Mutual Funds" list are some of the top-performing, best managed funds available. If you're already invested in them, congratulations! If you're not, don't worry - it's never too late to start getting the advantages of these outstanding funds for 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 learn about some of Zacks' highest ranked mutual funds with low fees you may want to consider.

Nicholas II Fund N (NNTWX - Free Report) has a 0.95% expense ratio and 0.52% management fee. NNTWX 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 yearly returns of 11.02% over the last five years, this fund clearly wins.

MFS New Discovery Fund I (MNDIX - Free Report): 1.05% expense ratio and 0.88% management fee. MNDIX 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. MNDIX, with annual returns of 13.44% over the last five years, is a well-diversified fund with a long track record of success.

Fidelity Series Opportunistic Insights (FVWSX - Free Report): 0.03% expense ratio and 0% management fee. FVWSX 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 a five-year annual return of 13.07%, this fund is a well-diversified fund with a long track record of success.

These examples highlight the fact that there are some astonishingly good mutual funds out there. If your advisor has you in the good ones, bravo! If not, you may need to have a talk.