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

Janus Henderson Global Life Science I (JFNIX Quick QuoteJFNIX - Free Report) : 0.76% expense ratio and 0.64% management fee. JFNIX is a Sector - Health mutual fund, which give investors an opportunity to focus on healthcare, one of the largest sectors of the American economy. With annual returns of 12.02% over the last five years, this fund is a winner.

Oberweis Small-Cap Opportunities (OBSOX Quick QuoteOBSOX - Free Report) : 1.25% expense ratio and 0.8% management fee. OBSOX is a Small Cap Growth mutual fund building their portfolio around stocks with market caps under $2 billion and large growth opportunities. OBSOX, with annual returns of 14.41% over the last five years, is a well-diversified fund with a long track record of success.

T. Rowe Price New Amer Growth I (PNAIX Quick QuotePNAIX - Free Report) . Expense ratio: 0.66%. Management fee: 0.63%. Five year annual return: 13.58%. PNAIX 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.

These examples highlight the fact that there are some astonishingly good mutual funds out there. If your advisor has you in the good ones, bravo! If not, you may need to have a talk.