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 learn about some of Zacks' highest ranked mutual funds with low fees you may want to consider.

T. Rowe Price All-Cap Opportunities Fund (PRWAX Quick QuotePRWAX - Free Report) : 0.81% expense ratio and 0.63% management fee. PRWAX 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. PRWAX has achieved five-year annual returns of an astounding 14.24%.

Bridgeway Small Cap Value N (BRSVX Quick QuoteBRSVX - Free Report) : 0.85% expense ratio and 0.61% management fee. BRSVX is a Small Cap Value fund, and these funds are known for investing in companies with market caps under $2 billion. With yearly returns of 11.29% over the last five years, BRSVX is an effectively diversified fund with a long reputation of solidly positive performance.

TIAA-CREF SocialChoice LwCrbnEq Ins (TNWCX Quick QuoteTNWCX - Free Report) . Expense ratio: 0.31%. Management fee: 0.25%. Five year annual return: 10.79%. TNWCX 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.

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.