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.

Vanguard Mid-Cap Growth Fund (VMGRX Quick QuoteVMGRX - Free Report) : 0.38% expense ratio and 0.34% management fee. VMGRX is a Mid Cap Growth mutual fund. These funds aim to target companies with a market capitalization between $2 billion and $10 billion that are also expected to exhibit more extensive growth opportunities for investors than their peers. With annual returns of 10.9% over the last five years, this fund is a winner.

Sextant International Fund (SSIFX Quick QuoteSSIFX - Free Report) . Expense ratio: 1.13%. Management fee: 0.5%. SSIFX is a Non US - Equity option, focusing their investments acoss emerging and developed markets, and can often extend across cap levels too. This fund has managed to produce a robust 10.56% over the last five years.

Undiscovered Managers Behavioral Value I (UBVSX Quick QuoteUBVSX - Free Report) : 0.99% expense ratio and 0.75% management fee. UBVSX is a Small Cap Value mutual fund option, which typically invest in companies with market caps under $2 billion. With a five-year annual return of 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.