There is never a wrong time to invest in mutual funds for retirement. So, if you're still looking for the best mutual funds, the Zacks Mutual Fund Rank can be a great guide.

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.

Direxion Mo S&P 500 Bull 1.75X Inv (DXSLX Quick QuoteDXSLX - Free Report) : 1.32% expense ratio and 0.75% management fee. DXSLX is classified as an Allocation Balanced fund, which seeks to invest in a balance of asset types, like stocks, bonds, and cash, and including precious metals or commodities is not unusual. DXSLX has achieved five-year annual returns of an astounding 18.87%.

WCM International SmCap Growth Inst (WCMSX Quick QuoteWCMSX - Free Report) : 1.25% expense ratio and 1% management fee. WCMSX is a Non US - Equity option, focusing their investments acoss emerging and developed markets, and can often extend across cap levels too. With yearly returns of 11.07% over the last five years, WCMSX is an effectively diversified fund with a long reputation of solidly positive performance.

VALIC Company I Large Cap Growth Fund (VLCGX Quick QuoteVLCGX - Free Report) . Expense ratio: 0.74%. Management fee: 0.64%. Five year annual return: 15.64%. VLCGX is a Large Cap Growth mutual fund, and these funds invest in many large U.S. firms that are projected to grow at a faster rate than their large-cap 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.