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.

Victory Munder Multi-Cap A (MNNAX Quick QuoteMNNAX - Free Report) : 1.21% expense ratio and 0.75% management fee. MNNAX is a Sector - Tech mutual fund, allowing investors to own a stake in a notoriously volatile sector with a much more diversified approach. With annual returns of 13.79% over the last five years, this fund is a winner.

Thornburg Investment Inc Builder R6 (TIBOX): 0.8% expense ratio and 0.69% management fee. TIBOX is an Allocation Balanced mutual fund. Allocation Balanced funds look to invest across asset types, like stocks, bonds, and cash, and including precious metals or commodities is not unusual; these funds are mostly categorized by their respective asset allocation. TIBOX, with annual returns of 13.84% over the last five years, is a well-diversified fund with a long track record of success.

Timothy Plan Aggressive Growth I (TIAGX). Expense ratio: 1.35%. Management fee: 0.85%. Five year annual return: 11.78%. TIAGX is a Mid Cap Growth mutual fund. These mutual funds choose companies with a stock market valuation between $2 billion and $10 billion.

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.