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.

Northern Large Cap Value (NOLVX Quick QuoteNOLVX - Free Report) : 0.57% expense ratio and 0.53% management fee. NOLVX 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. NOLVX has achieved five-year annual returns of an astounding 11.27%.

Pioneer Global Equity Y (PGSYX Quick QuotePGSYX - Free Report) : 0.72% expense ratio and 0.65% management fee. PGSYX is a Global - Equity mutual fund, which invests their assets in large markets, leveraging the global economy. PGSYX, with annual returns of 15.51% over the last five years, is a well-diversified fund with a long track record of success.

Undiscovered Managers Behavioral Value R6 (UBVFX Quick QuoteUBVFX - Free Report) . Expense ratio: 0.8%. Management fee: 0.75%. Five year annual return: 13.72%. UBVFX is a Small Cap Value fund, and these funds are known for investing in companies with market caps under $2 billion.

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.