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 learn about some of Zacks' highest ranked mutual funds with low fees you may want to consider.

Oppenheimer Discovery R (ODINX Quick QuoteODINX - Free Report) : 1.26% expense ratio and 0.59% management fee. ODINX is one of many Small Cap Growth mutual funds; these funds tend to create their portfolios around stocks with market capitalization of less than $2 billion. ODINX has achieved five-year annual returns of an astounding 12.95%.

MFS Mass Investors Growth Stock C (MIGDX Quick QuoteMIGDX - Free Report) : 1.45% expense ratio and 0.33% management fee. MIGDX 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 14.77% over the last five years, MIGDX is an effectively diversified fund with a long reputation of solidly positive performance.

Janus Henderson Enterprise T (JAENX Quick QuoteJAENX - Free Report) . Expense ratio: 0.9%. Management fee: 0.64%. Five year annual return: 12.48%. JAENX is a Mid Cap Growth mutual fund. These funds aim to target companies with a market capitalization between $2 billion and $10 billion that are also expected to exhibit more extensive growth opportunities for investors than their 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.