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.

If you are looking to diversify your portfolio, consider American Century Small Cap Value R6 (ASVDX Quick QuoteASVDX - Free Report) . ASVDX is a Small Cap Value fund, and these funds are known for investing in companies with market caps under $2 billion. This fund is a winner, boasting an expense ratio of 0.74%, management fee of 0.73%, and a five-year annualized return track record of 15%.

Voya Mid Cap Opportunities Portfolio S (ISMOX Quick QuoteISMOX - Free Report) : 0.91% expense ratio and 0.79% management fee. ISMOX 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. ISMOX, with annual returns of 13.42% over the last five years, is a well-diversified fund with a long track record of success.

Transamerica Large Cap Value I2 (TWQZX Quick QuoteTWQZX - Free Report) : 0.63% expense ratio and 0.58% management fee. TWQZX is a part of the Large Cap Value category, and invests in equities with a market capitalization of $10 billion or more, but whose share prices do not reflect their intrinsic value. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 10.4% 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.