There are many buy signals investors use. However, none is as powerful as purchases by insiders of stock in their own company.

In a 10-year study, University of Michigan professor H. Nejat Seyhun noted that insider buys beat the market by a full 3.3%. To give you some context, as Motley Fool banking analyst Anand Chookkavelu noted, doing this over an extended time period would make you one of the greatest investors of all time.

The biggest screaming buy this week
Informed by this knowledge, shares in megabank JPMorgan Chase (NYSE: JPM) may be the best buy in the market this week.

Late in the day on Friday, the bank disclosed that Jamie Dimon, its much-heralded chairman and CEO, purchased 500,000 shares of his employer's stock. At an average of price $34.27 a share, the purchase amounts to $17 million.

Now, I know what you're thinking. Dimon is a very wealthy man, and $17 million to him is not the same as $17 million to you or me. That's true. But it still amounts to nearly three-fourths of his entire 2011 compensation package of $23 million. In other words, it's not an inconsequential figure even to Dimon.

Shares in JPMorgan took a 32% plunge two months ago, falling from a high of nearly $44 per share down to $31, after the bank revealed that its London office had suffered massive trading losses in May. In what came to be known as the "London Whale" scandal, the losses were estimated originally at $1 billion but eventually ballooned to nearly five times that number, causing a $4.4 billion write off in its most recent quarter.

Despite this, JPMorgan crushed earnings estimates in the second quarter, logging earnings per share of $1.21 versus an expected $0.72. Shares in the nation's largest bank by assets are down 3% year to date and are trading for 70% of book value, leading Motley Fool analyst Jason Moser to say that it is one headline-risk stock worth owning today.

1 bank stock for your portfolio
For the identity of another great bank to consider adding to your portfolio, check out our free report, "The Stocks Only the Smartest Investors Are Buying," which profiles a much smaller bank that many are calling the best bank, not to mention bank stock, in America. Access this free report instantly.