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.

Let's break down some of the mutual funds with the top Zacks Mutual Fund Rank and the lowest fees.

American Funds Growth Fund of America R6 (RGAGX): 0.29% expense ratio and 0.25% management fee. RGAGX 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. RGAGX has achieved five-year annual returns of an astounding 10.83%.

Royce Micro-Cap Service Class (RMCFX). Expense ratio: 1.49%. Management fee: 1%. RMCFX is a Small Cap Value mutual fund option, which typically invest in companies with market caps under $2 billion. This fund has managed to produce a robust 10.54% over the last five years.

Goldman Sachs Enhncd Dvdnd Gl Eq P (GAFPX Quick QuoteGAFPX - Free Report) : 0.18% expense ratio and 0.15% management fee. GAFPX is a Global - Equity mutual fund. These funds invest in large markets like the U.S., Europe, and Japan, and operate with very few geographical limitations With a five-year annual return of 10.27%, this fund is a well-diversified fund with a long track record of success.

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.