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 take a look at some of our top-ranked mutual funds with the lowest fees.

MassMutual Premier Small/Mid Cap Opportunities Service Class

(MSVYX Quick QuoteMSVYX - Free Report) : 0.86% expense ratio and 0.58% management fee. MSVYX 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 annual returns of 10.26% over the last five years, this fund is a winner.

MFS Mid Cap Value R4

(MVCJX Quick QuoteMVCJX - Free Report) : 0.74% expense ratio and 0.61% management fee. MVCJX is a Mid Cap Value mutual fund, which targets medium-sized companies with a market cap between $2 billion and $10 billion. MVCJX, with annual returns of 10.22% over the last five years, is a well-diversified fund with a long track record of success.

Vanguard Strategic Equity Fund

(VSEQX Quick QuoteVSEQX - Free Report) : 0.17% expense ratio and 0.16% management fee. VSEQX is a Mid Cap Blend mutual fund, and usually features a portfolio with stocks of various styles and sizes, allowing for diversification within a strategy that focuses on mid cap companies. With a five-year annual return of 11.54%, this fund is a well-diversified fund with a long track record of success.

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.