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, most reliable way to judge a mutual fund's quality over time is by analyzing its performance, diversification, and fees. The Zacks Mutual Fund Rank, which covers over 19,000 mutual funds, has helped us identify three outstanding options that are perfect for any long-term investors' portfolios that is retirement-focused.

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

Thornburg Investment Income Builder R4 (TIBGX): 1.3% expense ratio and 0.69% management fee. TIBGX is classified as an Allocation Balanced fund, which seeks to invest in a balance of asset types, like stocks, bonds, and cash, and including precious metals or commodities is not unusual. With annual returns of 14.99% over the last five years, this fund is a winner.

American Funds International Growth & Income 529F (CGIFX Quick QuoteCGIFX - Free Report) : 0.73% expense ratio and 0.48% management fee. CGIFX is a Non US - Equity option, focusing their investments acoss emerging and developed markets, and can often extend across cap levels too. With yearly returns of 11.18% over the last five years, CGIFX is an effectively diversified fund with a long reputation of solidly positive performance.

Sterling Capital Bhv LgCapVal Eq R6 (STRAX): 0.7% expense ratio and 0.45% management fee. STRAX is a Large Cap Value mutual fund, which invests in stocks with a market cap of $10 billion of more, but whose share prices do not reflect their intrinsic value. With a five-year annual return of 17.76%, 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.