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

Fidelity Value Fund (FDVLX Quick QuoteFDVLX - Free Report) has a 0.86% expense ratio and 0.73% management fee. FDVLX is a Mid Cap Value mutual funds that aims to target medium-sized companies that possess strong value and income opportunities for investors. With yearly returns of 16.22% over the last five years, this fund clearly wins.

BlackRock Basic Value A (MDBAX Quick QuoteMDBAX - Free Report) : 0.83% expense ratio and 0.42% management fee. MDBAX 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. MDBAX, with annual returns of 11.45% over the last five years, is a well-diversified fund with a long track record of success.

Columbia Small Cap Value II Class Z (NSVAX Quick QuoteNSVAX - Free Report) is an attractive large-cap allocation. NSVAX is a Small Cap Value mutual fund option, which typically invest in companies with market caps under $2 billion. NSVAX has an expense ratio of 1.01%, management fee of 0.83%, and annual returns of 11.89% 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.