NEW YORK (CNN/Money) - After the boom and bust of the past five years, investors have grown suspicious of stocks with great stories but unimpressive profits. And they've become skeptical of star CEOs as well.

The skepticism is understandable -- just look at what has happened to Martha Stewart's stock. But investors shouldn't automatically dismiss the contributions that a dynamic CEO can make to a company's success.

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There's no way you can actually quantify a CEO's contribution. Sheer dynamism by itself doesn't count for much -- the days when investors were wowed by footage of Lee Iacocca striding purposefully around are long gone.

So what should investors be looking for? What specific things can a CEO contribute that will translate into investing profits?

In fact, every star CEO contributes in a different way. Jack Welch managed to maintain a rigorous management style at General Electric that enabled the multibillion-dollar giant to expand like a nimble, mid-size growth stock. Martha Stewart, by contrast, created her company in her own image. The great vulnerability there is that once she was tainted, so was her stock.

And sometimes, a CEO's contributions don't pay off until after he is gone. Citigroup's share price was actually lower in March 2003 than it was in October 1999. It's only since the recovery began that Weill's efforts have begun to show up in the stock price. Moreover, Weill's successor as CEO, Charles Prince, has announced that he plans to focus on bringing the gains in assets down to the bottom line. So the real payoff for Weill's successful deals may still lie in the future.

Even if a CEO's contribution can't be quantified, you do know it when you see it.

Here are my nominations for three corporate leaders who have fashioned a whole that's far more than the sum of its parts.

Founded by Brian Roberts' father Ralph, Comcast has grown from a mid-sized cable company to the industry's 800-pound gorilla, thanks to bold acquisitions. The company now has more than 21 million subscribers, making it 65 percent bigger than No. 2 Time Warner (owner of this Web site).

Comcast's (CMCSK: Research, Estimates) boldest move was its acquisition of AT&T Broadband for $50.5 billion in 2002. And now, with more than 5 million Internet subscribers, Comcast is also the leading U.S. broadband operator.

Most recently, Roberts bid more than $26 a share for Disney, hoping to add content and the ABC network to Comcast's cable distribution. The bid, which was not particularly generous, has been refused.

But Roberts has always been patient and disciplined in his dealmaking. And he is waiting, hoping for the chance to complete the Disney offer -- or find some other deal -- that will finally make Comcast the equal of the other diversified media giants.

When Vivendi Universal began to unravel, Barry Diller was one of the few insiders to escape with his assets intact. Essentially, he walked away with the foundering media company's online businesses.

Since then, he has built them into an extraordinary collection of e-commerce businesses named InterActiveCorp (IACI: Research, Estimates). It includes the HSN Home Shopping Network, the Expedia Travel service, Ticketmaster, online dating service Match.com and the like. Most recently, Diller bought ZeroDegrees for an undisclosed amount. The so-called social-networking service, which is less than a year old, enables business people to recruit employees and seek financing online.

Analysts who have followed Diller's career say that his greatest strength is the ability to spot Internet companies with business models that will eventually produce significant bottom-line income.

During the time that he was CEO of former cable giant TeleCommunications Inc., John Malone took stakes in many of the channels that were carried by his cable systems. TCI was sold to AT&T, most of which was subsequently acquired by Comcast.

Once all the complex transactions were over, Malone's cable channel interests were owned by the independent company, Liberty Media. In addition, Liberty owns stakes in other media companies, such as News Corp. and Time Warner. Malone is no longer CEO of the company, but as Chairman, he continues to set its long-term course.

Liberty's (L: Research, Estimates) stock currently trades at $11.25 a share, less than 20 percent of its 2000 peak. Analysts say that Liberty is now considering several deals -- from asset swaps to a full break-up -- that would recognize some of the company's hidden value. The company's total asset value is estimated at around $17 a share, and a deal could add two or three dollars to Liberty's current share price fairly quickly.

Michael Sivy is an editor-at-large for MONEY magazine. Sign up for free e-mail delivery every Tuesday and Thursday of Sivy on Stocks.