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 way to judge a mutual fund's quality over time is by analyzing its performance, diversification, and fees. Using the Zacks Mutual Fund Rank of over 19,000 mutual funds, we've identified three outstanding mutual funds that are ideally suited to help long-term investors pursue and achieve their retirement investing goals.

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

MFS Growth Fund R4 (MFEJX) has a 0.61% expense ratio and 0.48% management fee. MFEJX 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. With yearly returns of 10.4% over the last five years, this fund clearly wins.

Oberweis Small-Cap Opportunities (OBSOX). 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 13.28% over the last five years.

Fidelity Advisor Semiconductors M (FELTX Quick QuoteFELTX - Free Report) : 1.19% expense ratio and 0.67% management fee. With a much more diversified approach, FELTX--part of the Sector - Tech mutual fund category--gives investors a way to own a stake in the notoriously risky tech sector. The fund is mainly invested in equities, has a long reputation of salutary performance, and has yearly returns of 30.5% over the last 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.