Investing in mutual funds for retirement is never too late. And the Zacks Mutual Fund Rank can be an excellent tool for investors looking to invest in the best funds.

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 retirement investors' portfolios.

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

Fidelity Value Fund (FDVLX Quick QuoteFDVLX - Free Report) : 0.87% expense ratio and 0.73% management fee. FDVLX is a Mid Cap Value fund, which usually invests in companies with a stock market valuation between $2 billion and $10 billion. With annual returns of 15.58% over the last five years, this fund is a winner.

Neuberger Berman Mid Cap Growth Adviser (NBMBX Quick QuoteNBMBX - Free Report) . Expense ratio: 1.19%. Management fee: 0.89%. NBMBX 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. This fund has managed to produce a robust 10.64% over the last five years.

Invesco Discovery A (OPOCX Quick QuoteOPOCX - Free Report) : 1.06% expense ratio and 0.6% management fee. OPOCX is a Small Cap Growth mutual fund building their portfolio around stocks with market caps under $2 billion and large growth opportunities. With a five-year annual return of 11.88%, 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.