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

Invesco Comstock Y (ACSDX Quick QuoteACSDX - Free Report) : 0.56% expense ratio and 0.38% management fee. ACSDX 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. With annual returns of 13.68% over the last five years, this fund is a winner.

BlackRock Natural Resources Institutional (MAGRX Quick QuoteMAGRX - Free Report) . Expense ratio: 0.83%. Management fee: 0.6%. MAGRX is a Sector - Energy mutual fund, which encompasses a wide range of vastly changing and vitally important industries throughout this massive global sector. This fund has managed to produce a robust 12.54% over the last five years.

Principal Mid Cap Value I R4 (PABWX Quick QuotePABWX - Free Report) : 1% expense ratio and 0.64% management fee. PABWX is a Mid Cap Value fund, which usually invests in companies with a stock market valuation between $2 billion and $10 billion. With a five-year annual return of 12.76%, this fund is a well-diversified fund with a long track record of success.

We hope that your investment advisor (if you use one) has you invested in one or all of the top-ranked mutual funds we've reviewed. But if that isn't the case, it might be time to have a conversation or reconsider this vitally important relationship.