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 retirement investors' portfolios.

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

Neuberger Berman Guardian Institutional (NGDLX Quick QuoteNGDLX - Free Report) : 0.68% expense ratio and 0.63% management fee. NGDLX is a part of the Large Cap Value category, and invests in equities with a market capitalization of $10 billion or more, but whose share prices do not reflect their intrinsic value. NGDLX has achieved five-year annual returns of an astounding 12.34%.

Oppenheimer Discovery R (ODINX Quick QuoteODINX - Free Report) . Expense ratio: 1.28%. Management fee: 0.59%. ODINX is a Small Cap Growth mutual fund building their portfolio around stocks with market caps under $2 billion and large growth opportunities. This fund has managed to produce a robust 11.02% over the last five years.

Great-WestMultiMgrLgCapGrowIns (MXGSX Quick QuoteMXGSX - Free Report) : 0.65% expense ratio and 0.64% management fee. MXGSX is a part of the Large Cap Growth mutual fund category, which invest in many large U.S. companies that are expected to grow much faster compared to other large-cap stocks. With a five-year annual return of 14.14%, this fund is a well-diversified fund with a long track record of success.

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.