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.

How can you tell a good mutual fund from a bad one? It's pretty basic: if the fund is diversified, has low fees, and shows strong performance, it's a keeper. Of course, there's a wide range, but using the Zacks Mutual Fund Rank, we've found three mutual funds that would be great additions to any long-term investors' portfolios.

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

Nuveen Core Equity A (TIIRX): 0.68% expense ratio and 0.39% management fee. TIIRX 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. TIIRX has achieved five-year annual returns of an astounding 11.11%.

FullerThaler Behav SmCap Grow Inst (FTXSX): 0.99% expense ratio and 0.85% management fee. FTXSX is a Small Cap Growth mutual fund building their portfolio around stocks with market caps under $2 billion and large growth opportunities. With yearly returns of 11.89% over the last five years, FTXSX is an effectively diversified fund with a long reputation of solidly positive performance.

MFS Blended Research Intl Eq R2 (BRXSX Quick QuoteBRXSX - Free Report) is an attractive large-cap allocation. BRXSX is a part of the Non US - Equity fund category, many of which will focus across all cap levels, and will typically allocate their investments between emerging and developed markets. BRXSX has an expense ratio of 1%, management fee of 0.35%, and annual returns of 12.66% over the past five years.

These examples highlight the fact that there are some astonishingly good mutual funds out there. If your advisor has you in the good ones, bravo! If not, you may need to have a talk.