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.

Here are the funds that have achieved the Zacks Mutual Fund Rank #1 (Strong Buy) and have low fees.

BNY Mellon Natural Resources I (DLDRX Quick QuoteDLDRX - Free Report) has a 0.91% expense ratio and 0.75% management fee. DLDRX is a Sector - Energy mutual fund, which encompasses a wide range of vastly changing and vitally important industries throughout this massive global sector. With yearly returns of 16.83% over the last five years, this fund clearly wins.

Fidelity Select Tech Hardware (FDCPX Quick QuoteFDCPX - Free Report) : 0.72% expense ratio and 0.53% management fee. FDCPX is a Sector - Tech mutual fund, allowing investors to own a stake in a notoriously volatile sector with a much more diversified approach. FDCPX, with annual returns of 12.98% over the last five years, is a well-diversified fund with a long track record of success.

MFS Mass Investors Growth Stock A (MIGFX Quick QuoteMIGFX - Free Report) . Expense ratio: 0.71%. Management fee: 0.33%. Five year annual return: 13.38%. MIGFX 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.