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.

ARGA International Value Instl (ARVIX Quick QuoteARVIX - Free Report) : 0.75% expense ratio and 0.6% management fee. ARVIX is a part of the Non US - Equity fund category, many of which will focus across all cap levels, and will typically allocate their investments between emerging and developed markets. ARVIX has achieved five-year annual returns of an astounding 15.45%.

Nuveen Quant SMid-Cap Eq W (TSMUX): 0.46% expense ratio and 0.42% management fee. TSMUX is a Small Cap Blend mutual fund, allowing investors a way to diversify their funds among various types of small-cap stocks. With yearly returns of 13.81% over the last five years, TSMUX is an effectively diversified fund with a long reputation of solidly positive performance.

Calamos Timpani Small Growth R6 (CTSOX): 0.95% expense ratio and 0.9% management fee. CTSOX is a Small Cap Growth mutual fund and tends to feature small companies in up-and-coming industries and markets. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 10.36% 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.