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 retirement investors' portfolios.

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

If you are looking to diversify your portfolio, consider Janus Henderson Contrarian S (JCNIX Quick QuoteJCNIX - Free Report) . JCNIX is a Large Cap Value fund. These funds invest in stocks with a market cap of $10 billion of more, but whose share prices do not reflect their intrinsic value. This fund is a winner, boasting an expense ratio of 1.21%, management fee of 0.6%, and a five-year annualized return track record of 13.6%.

Principal Mid Cap R5 (PMBPX Quick QuotePMBPX - Free Report) : 0.84% expense ratio and 0.58% management fee. PMBPX is a Mid Cap Growth mutual fund. These funds aim to target companies with a market capitalization between $2 billion and $10 billion that are also expected to exhibit more extensive growth opportunities for investors than their peers. PMBPX, with annual returns of 13.57% over the last five years, is a well-diversified fund with a long track record of success.

Voya Large Cap Growth A (NLCAX Quick QuoteNLCAX - Free Report) : 0.94% expense ratio and 0.51% management fee. NLCAX 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. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 14.51% over the last five years.

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.