It is never too late to invest in mutual funds for retirement. As such, if you plan to invest in some of the best funds, the Zacks Mutual Fund Rank can provide you with valuable guidance.

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.

AB International Strategy Equities Adviser (STEYX): 0.68% expense ratio and 0.66% management fee. STEYX 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. With annual returns of 11.41% over the last five years, this fund is a winner.

Nuveen Quant SmallCap Equity Retire (TRSEX): 0.65% expense ratio and 0.39% management fee. TRSEX is a Small Cap Blend mutual fund, and usually targets stocks with market caps of less than $2 billion, letting investors diversify their funds among other kinds of small-cap equities. TRSEX, with annual returns of 9.72% over the last five years, is a well-diversified fund with a long track record of success.

BlackRock Advantage Large Cap Growth I (CMVIX Quick QuoteCMVIX - Free Report) . Expense ratio: 0.62%. Management fee: 0.56%. Five year annual return: 11.29%. CMVIX is a Large Cap Growth mutual fund, and these funds invest in many large U.S. firms that are projected to grow at a faster rate than their large-cap peers.

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.