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.

Neuberger Berman Guardian A (NGDAX Quick QuoteNGDAX - Free Report) : 1.05% expense ratio and 0.74% management fee. NGDAX is a Large Cap Value mutual fund, which invests in stocks with a market cap of $10 billion of more, but whose share prices do not reflect their intrinsic value. NGDAX has achieved five-year annual returns of an astounding 13.14%.

Invesco Small Cap Value Y (VSMIX Quick QuoteVSMIX - Free Report) : 0.83% expense ratio and 0.64% management fee. VSMIX 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 10% over the last five years, VSMIX is an effectively diversified fund with a long reputation of solidly positive performance.

AB Lg Cap Growth Z (APGZX Quick QuoteAPGZX - Free Report) . Expense ratio: 0.51%. Management fee: 0.48%. Five year annual return: 13.35%. APGZX 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.