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.

Elfun Trusts (ELFNX Quick QuoteELFNX - Free Report) : 0.18% expense ratio and 0.14% management fee. ELFNX is classified as a Large Cap Blend fund. More often than not, Large Cap Blend mutual funds invest in companies with a market cap of over $10 billion. Buying stakes in bigger companies offer these funds more stability, and are well-suited for investors with a "buy and hold" mindset. ELFNX has achieved five-year annual returns of an astounding 18.39%.

Invesco Discovery A (OPOCX Quick QuoteOPOCX - Free Report) : 1.06% expense ratio and 0.6% management fee. OPOCX is one of many Small Cap Growth mutual funds; these funds tend to create their portfolios around stocks with market capitalization of less than $2 billion. OPOCX, with annual returns of 11.88% over the last five years, is a well-diversified fund with a long track record of success.

Neuberger Berman Mid Cap Growth R6 (NRMGX Quick QuoteNRMGX - Free Report) . Expense ratio: 0.59%. Management fee: 0.54%. Five year annual return: 10.2%. NRMGX 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.

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.