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 break down some of the mutual funds with the top Zacks Mutual Fund Rank and the lowest fees.

Goldman Sachs Large Cap Growth Insights IR (GLCTX Quick QuoteGLCTX - Free Report) : 0.68% expense ratio and 0.51% management fee. GLCTX is a Large Cap Growth option; these mutual funds purchase stakes in numerous large U.S. companies that are expected to develop and grow at a faster rate than other large-cap stocks. With annual returns of 16.39% over the last five years, this fund is a winner.

John Hancock Disciplined Value I (JVLIX Quick QuoteJVLIX - Free Report) : 0.7% expense ratio and 0.61% management fee. JVLIX 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 yearly returns of 12.03% over the last five years, JVLIX is an effectively diversified fund with a long reputation of solidly positive performance.

T. Rowe Price New Horizons I (PRJIX Quick QuotePRJIX - Free Report) : 0.66% expense ratio and 0.64% management fee. PRJIX 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. With a five-year annual return of 12.88%, this fund is a well-diversified fund with a long track record of success.

We hope that your investment advisor (if you use one) has you invested in one or all of the top-ranked mutual funds we've reviewed. But if that isn't the case, it might be time to have a conversation or reconsider this vitally important relationship.