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

DWS Science and Technology A (KTCAX Quick QuoteKTCAX - Free Report) has a 0.88% expense ratio and 0.45% management fee. KTCAX is a Sector - Tech mutual fund, allowing investors to own a stake in a notoriously volatile sector with a much more diversified approach. With yearly returns of 14.48% over the last five years, this fund clearly wins.

PIMCO Stock Plus International (US Dollar-Hedged) I (PISIX): 0.79% expense ratio and 0.75% management fee. PISIX is a part of the Non US - Equity fund category, many of which will focus across all cap levels, and will typically allocate their investments between emerging and developed markets. PISIX, with annual returns of 14.7% over the last five years, is a well-diversified fund with a long track record of success.

Nuveen Quant SMid-Cap Eq R6 (TSMWX) is an attractive large-cap allocation. TSMWX is a Small Cap Blend mutual fund, and usually targets stocks with market caps of less than $2 billion, letting investors diversify their funds among other kinds of small-cap equities. TSMWX has an expense ratio of 0.46%, management fee of 0.43%, and annual returns of 11.71% over the past five years.

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.