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 break down some of the mutual funds with the top Zacks Mutual Fund Rank and the lowest fees.

Bridgeway Small Cap Value N (BRSVX Quick QuoteBRSVX - Free Report) : 0.83% 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. BRSVX has achieved five-year annual returns of an astounding 13.47%.

Elfun Trusts (ELFNX Quick QuoteELFNX - Free Report) . Expense ratio: 0.18%. Management fee: 0.14%. ELFNX is part of the Large Cap Blend section, and these mutual funds most often invest in firms with a market capitalization of $10 billion or more. By investing in bigger companies, these funds offer more stability, and are often well-suited for investors with a "buy and hold" mindset. This fund has managed to produce a robust 10.54% over the last five years.

MFS Mass Investors Growth Stock C (MIGDX Quick QuoteMIGDX - Free Report) . Expense ratio: 1.46%. Management fee: 0.33%. Five year annual return: 11.06%. MIGDX 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.

We hope that your investment advisor (if you use one) has you invested in one or all of the top-ranked mutual funds we've reviewed. But if that isn't the case, it might be time to have a conversation or reconsider this vitally important relationship.