There is never a wrong time to invest in mutual funds for retirement. So, if you're still looking for the best mutual funds, the Zacks Mutual Fund Rank can be a great guide.

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 retirement investors' portfolios.

Let's learn about some of Zacks' highest ranked mutual funds with low fees you may want to consider.

QS Global Equity A

(CFIPX Quick QuoteCFIPX - Free Report) : 1.29% expense ratio and 0.75% management fee. CFIPX is a Non US - Equity option, focusing their investments acoss emerging and developed markets, and can often extend across cap levels too. With annual returns of 13.31% over the last five years, this fund is a winner.

Janus Henderson Research D

(JNRFX Quick QuoteJNRFX - Free Report) is a stand out amongst its peers. JNRFX is a part of the Large Cap Growth mutual fund category, which invest in many large U.S. companies that are expected to grow much faster compared to other large-cap stocks. With five-year annualized performance of 16.23%, expense ratio of 0.64% and management fee of 0.48%, this diversified fund is an attractive buy with a strong history of performance.

Principal Mid Cap R4

(PMBSX Quick QuotePMBSX - Free Report) . Expense ratio: 0.96%. Management fee: 0.58%. Five year annual return: 11.8%. PMBSX is a Mid Cap Growth mutual fund. These mutual funds choose companies with a stock market valuation between $2 billion and $10 billion.

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.