Investing in mutual funds for retirement is never too late. And the Zacks Mutual Fund Rank can be an excellent tool for investors looking to invest in the best funds.

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.

Brown Advisory Flexible Equity Institutional (BAFFX Quick QuoteBAFFX - Free Report) : 0.54% expense ratio and 0.43% management fee. BAFFX is an Allocation Balanced mutual fund. Allocation Balanced funds look to invest across asset types, like stocks, bonds, and cash, and including precious metals or commodities is not unusual; these funds are mostly categorized by their respective asset allocation. BAFFX has achieved five-year annual returns of an astounding 12.28%.

Heartland Mid Cap Value Instl (HNMDX Quick QuoteHNMDX - Free Report) is a stand out amongst its peers. HNMDX is a Mid Cap Value fund, which usually invests in companies with a stock market valuation between $2 billion and $10 billion. With five-year annualized performance of 10.66%, expense ratio of 0.85% and management fee of 0.75%, this diversified fund is an attractive buy with a strong history of performance.

Wells Fargo Large Cap Growth R6 (STFFX Quick QuoteSTFFX - Free Report) : 0.74% expense ratio and 0.69% management fee. STFFX is a Large Cap Growth option; these mutual funds purchase stakes in numerous large U.S. companies that are expected to develop and grow at a faster rate than other large-cap stocks. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 10.32% over the last five years.

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.