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 investors' portfolios.

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

DFA US Micro Cap Institutional (DFSCX Quick QuoteDFSCX - Free Report) : 0.42% expense ratio and 0.38% management fee. DFSCX is a Small Cap Blend mutual fund that usually targets companies with a market capitalization of less than $2 billion. With annual returns of 11.13% over the last five years, this fund is a winner.

JPMorgan US GARP Equity I (JPGSX). Expense ratio: 0.59%. Management fee: 0.3%. JPGSX 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. This fund has managed to produce a robust 16.65% over the last five years.

MFS Blended Research Core Equity I (MUSEX) is an attractive large-cap allocation. MUSEX 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. MUSEX has an expense ratio of 0.49%, management fee of 0.25%, and annual returns of 15.68% over the past five years.

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.