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.

The easiest, most reliable way to judge a mutual fund's quality over time is by analyzing its performance, diversification, and fees. The Zacks Mutual Fund Rank, which covers over 19,000 mutual funds, has helped us identify three outstanding options that are perfect for any long-term investors' portfolios that is retirement-focused.

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

If you are looking to diversify your portfolio, consider Brown Advisory Sustainable Growth Adviser (BAWAX Quick QuoteBAWAX - Free Report) . BAWAX is a Large Cap Growth option; these mutual funds purchase stakes in numerous large U.S. companies that are expected to develop and grow at a faster rate than other large-cap stocks. This fund is a winner, boasting an expense ratio of 1.03%, management fee of 0.53%, and a five-year annualized return track record of 12.82%.

Fidelity Advisor Technology A (FADTX Quick QuoteFADTX - Free Report) : 0.97% expense ratio and 0.52% management fee. FADTX is a Sector - Tech mutual fund, allowing investors to own a stake in a notoriously volatile sector with a much more diversified approach. FADTX, with annual returns of 14.85% over the last five years, is a well-diversified fund with a long track record of success.

T. Rowe Price New Horizons I (PRJIX Quick QuotePRJIX - Free Report) is an attractive large-cap allocation. PRJIX is a Small Cap Growth mutual fund and tends to feature small companies in up-and-coming industries and markets. PRJIX has an expense ratio of 0.65%, management fee of 0.63%, and annual returns of 10.89% 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.