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.

Frost Growth Equity Institutional (FICEX Quick QuoteFICEX - Free Report) : 0.63% expense ratio and 0.5% management fee. FICEX 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. FICEX has achieved five-year annual returns of an astounding 10.75%.

Oberweis Small-Cap Opportunities (OBSOX Quick QuoteOBSOX - Free Report) . Expense ratio: 1.25%. Management fee: 0.8%. OBSOX is a Small Cap Growth mutual fund and tends to feature small companies in up-and-coming industries and markets. This fund has managed to produce a robust 15.48% over the last five years.

ProFunds Semicond UltraSector Investor (SMPIX Quick QuoteSMPIX - Free Report) : 1.49% expense ratio and 0.75% management fee. SMPIX is part of the Sector - Tech mutual fund category that invests in technology and lets investors own a stake in a notoriously volatile sector, but with a much more diversified approach. With a five-year annual return of 11.75%, this fund is a well-diversified fund with a long track record of success.

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.