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.

How can you tell a good mutual fund from a bad one? It's pretty basic: if the fund is diversified, has low fees, and shows strong performance, it's a keeper. Of course, there's a wide range, but using the Zacks Mutual Fund Rank, we've found three mutual funds that would be great additions to any long-term investors' portfolios.

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

FullerThaler Behav SmCap Grow A (FTXAX Quick QuoteFTXAX - Free Report) : 1.32% expense ratio and 0.85% management fee. FTXAX 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. FTXAX has achieved five-year annual returns of an astounding 10.99%.

Empower International Value Instl (MXJVX): 0.71% expense ratio and 0.67% management fee. MXJVX is a Non US - Equity option, focusing their investments acoss emerging and developed markets, and can often extend across cap levels too. MXJVX, with annual returns of 12.84% over the last five years, is a well-diversified fund with a long track record of success.

BlackRock Equity Dividend K (MKDVX): 0.58% expense ratio and 0.56% management fee. MKDVX is a part of the Large Cap Value category, and invests in equities with a market capitalization of $10 billion or more, but whose share prices do not reflect their intrinsic value. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 11.96% over the last five years.

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.