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

Goldman Sachs US Equity Insights I

(GSELX Quick QuoteGSELX - Free Report) has a 0.56% expense ratio and 0.52% management fee. GSELX is a Large Cap Blend fund, targeting companies with market caps of over $10 billion. These funds offer investors a stability, and are perfect for people with a "buy and hold" mindset. With yearly returns of 15.07% over the last five years, this fund clearly wins.

Janus Henderson Contrarian I

(JCONX Quick QuoteJCONX - Free Report) : 0.62% expense ratio and 0.49% management fee. JCONX is a Large Cap Value fund. These funds invest in stocks with a market cap of $10 billion of more, but whose share prices do not reflect their intrinsic value. With yearly returns of 13.48% over the last five years, JCONX is an effectively diversified fund with a long reputation of solidly positive performance.

Principal Mid Cap R3

(PMBMX Quick QuotePMBMX - Free Report) . Expense ratio: 1.14%. Management fee: 0.58%. Five year annual return: 11.34%. PMBMX is a Mid Cap Growth mutual fund. Mid Cap Growth funds pick stocks--usually companies with a market cap between $2 billion and $10 billion--that demonstrate extensive growth opportunities for investors compared to their peers.

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.