It is never too late to invest in mutual funds for retirement. As such, if you plan to invest in some of the best funds, the Zacks Mutual Fund Rank can provide you with valuable guidance.

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

Fidelity Advisor Mega Cap Stock I (FTRIX): 0.59% expense ratio and 0.58% management fee. FTRIX is classified as a Large Cap Blend fund. More often than not, Large Cap Blend mutual funds invest in companies with a market cap of over $10 billion. Buying stakes in bigger companies offer these funds more stability, and are well-suited for investors with a "buy and hold" mindset. FTRIX has achieved five-year annual returns of an astounding 21.38%.

Ariel Fund Investor (ARGFX Quick QuoteARGFX - Free Report) . Expense ratio: 0.99%. Management fee: 0.58%. ARGFX is a Mid Cap Value mutual fund, which targets medium-sized companies with a market cap between $2 billion and $10 billion. This fund has managed to produce a robust 14.8% over the last five years.

American Century Small Company R (ASCRX) is an attractive large-cap allocation. ASCRX is a Small Cap Blend mutual fund, allowing investors a way to diversify their funds among various types of small-cap stocks. ASCRX has an expense ratio of 1.35%, management fee of 0.85%, and annual returns of 10.52% 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.