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.

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

Oberweis Small-Cap Opps Instl (OBSIX) has a 1% expense ratio and 0.8% management fee. OBSIX is one of many Small Cap Growth mutual funds; these funds tend to create their portfolios around stocks with market capitalization of less than $2 billion. With yearly returns of 15.14% over the last five years, this fund clearly wins.

Sterling Capital Behavioral Large Cap Value Equity A (BBTGX Quick QuoteBBTGX - Free Report) : 1.05% expense ratio and 0.45% management fee. BBTGX is a Large Cap Value mutual fund, which invests in stocks with a market cap of $10 billion of more, but whose share prices do not reflect their intrinsic value. BBTGX, with annual returns of 13.67% over the last five years, is a well-diversified fund with a long track record of success.

Allspring Discipl US Core A (EVSAX) is an attractive large-cap allocation. EVSAX is a Large Cap Blend fund, targeting companies with market caps of over $10 billion. These funds offer investors a stability, and are perfect for people with a "buy and hold" mindset. EVSAX has an expense ratio of 0.84%, management fee of 0.35%, and annual returns of 14% over the past five years.

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.