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 best way to shortlist great mutual funds is to ensure solid performance, diversification, and low fees. Some are better than others, but utilizing the Zacks Mutual Fund Rank, we have identified three mutual funds that could be solid additions to one's retirement portfolio.

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

Buffalo Small Cap Fund (BUFSX Quick QuoteBUFSX - Free Report) : 1.01% expense ratio and 0.85% management fee. BUFSX is a Small Cap Growth mutual fund and tends to feature small companies in up-and-coming industries and markets. BUFSX has achieved five-year annual returns of an astounding 11.67%.

Fidelity Advisor Technology I (FATIX Quick QuoteFATIX - Free Report) : 0.71% expense ratio and 0.52% management fee. With a much more diversified approach, FATIX--part of the Sector - Tech mutual fund category--gives investors a way to own a stake in the notoriously risky tech sector. With yearly returns of 15.15% over the last five years, FATIX is an effectively diversified fund with a long reputation of solidly positive performance.

MFS Mass Investors Growth Stock I (MGTIX Quick QuoteMGTIX - Free Report) is an attractive large-cap allocation. MGTIX 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. MGTIX has an expense ratio of 0.46%, management fee of 0.33%, and annual returns of 13.66% 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.