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.

Here are the funds that have achieved the Zacks Mutual Fund Rank #1 (Strong Buy) and have low fees.

Vanguard Market Neutral Investor (VMNFX): 0.2% expense ratio and 0.17% management fee. VMNFX is a Market Neutral - Equity mutual fund. These funds attempt to maximize returns, and usually hold 50% of their securities in a long position and 50% in a short position. With annual returns of 11.83% over the last five years, this fund is a winner.

PGIM QuantSolutions Intl Equity R6 (PJRQX Quick QuotePJRQX - Free Report) . Expense ratio: 0.78%. Management fee: 0.75%. PJRQX 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. This fund has managed to produce a robust 12.7% over the last five years.

Putnam Research Fund R6 (PLJMX) is an attractive large-cap allocation. PLJMX is classified as a Large Cap Blend fund. More often than not, Large Cap Blend mutual funds invest in companies with a market cap of over $10 billion. Buying stakes in bigger companies offer these funds more stability, and are well-suited for investors with a "buy and hold" mindset. PLJMX has an expense ratio of 0.61%, management fee of 0.53%, and annual returns of 15.51% over the past 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.