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 investors' portfolios.

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

GMO U.S. Equity Allocation Fund Class VI (GMCQX): 0.4% expense ratio and 0.37% management fee. GMCQX 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 15.54% over the last five years, this fund is a winner.

MFS Blended Research Intl Eq R6 (BRXVX Quick QuoteBRXVX - Free Report) : 0.41% expense ratio and 0.35% management fee. BRXVX 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. BRXVX, with annual returns of 13.71% over the last five years, is a well-diversified fund with a long track record of success.

Victory Pioneer Global Equity K (PGEKX). Expense ratio: 0.73%. Management fee: 0.65%. Five year annual return: 15.51%. PGEKX is a Global - Equity mutual fund. These funds invest in large markets like the U.S., Europe, and Japan, and operate with very few geographical limitations

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.