Investing in mutual funds for retirement is never too late. And the Zacks Mutual Fund Rank can be an excellent tool for investors looking to invest in the best funds.

The easiest way to judge a mutual fund's quality over time is by analyzing its performance, diversification, and fees. Using the Zacks Mutual Fund Rank of over 19,000 mutual funds, we've identified three outstanding mutual funds that are ideally suited to help long-term investors pursue and achieve their retirement investing goals.

Let's break down some of the mutual funds with the top Zacks Mutual Fund Rank and the lowest fees.

If you are looking to diversify your portfolio, consider Neuberger Berman Mid Cap Growth Investor (NMANX Quick QuoteNMANX - Free Report) . NMANX is a Mid Cap Growth mutual fund. These mutual funds choose companies with a stock market valuation between $2 billion and $10 billion. This fund is a winner, boasting an expense ratio of 0.83%, management fee of 0.75%, and a five-year annualized return track record of 9.57%.

AQR Small Cap Multi-Style I (QSMLX): 0.62% expense ratio and 0.45% management fee. QSMLX is a Small Cap Blend mutual fund, and usually targets stocks with market caps of less than $2 billion, letting investors diversify their funds among other kinds of small-cap equities. With yearly returns of 14.81% over the last five years, QSMLX is an effectively diversified fund with a long reputation of solidly positive performance.

Vericimetry U.S. Small Cap Value Fund (VYSVX). Expense ratio: 0.6%. Management fee: 0.5%. Five year annual return: 17.08%. VYSVX is a Small Cap Value mutual fund option, which typically invest in companies with market caps under $2 billion.

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.