It doesn't take a genius to recall that ExxonMobil (NYSE: XOM) is the biggest of Big Oil. So with that fact out of the way, those with a yen for investing in energy are left to question whether the company is also the best of its group.

Let's defer judgment on the second issue until we've had an opportunity to reexamine the company's results for 2009 and to look at its plans for the future. After all, I have a soft spot for BP (NYSE: BP), a company that's improving at nearly breakneck speed. And beyond that, the likes of Chevron (NYSE: CVX) and Total (NYSE: TOT) continue to impress regularly. But an examination of Exxon's accomplishments for the year just past indicates a company that pretty much got it right most of the time.

The metrics that matter
For instance -- looking at a key metric for oil and gas companies -- Exxon replaced more than 133% of its production last year. As you know, the ability to increase total reserves from year to year is a key to separating the wheat from the chaff among energy producers and this marked Exxon's 16th straight year accomplishing a 100% or greater replacement figure. At the same time, its return on capital employed reached 16.3%, outdistancing all of its competitors by at least 50%. And it chalked up a 64% success rate for the 45 wildcat wells it drilled during the year. 

I could go on, but as I say frequently to my Foolish friends, investing is about looking ahead, not about information from our rear view mirrors. It therefore seems  noteworthy that, as last week wound down, CEO Rex Tillerson told a gathering of analysts that Exxon's capex would likely climb by about 4% to $28 billion. That really wasn't much of a surprise: Exxon had already pegged this year's level in the $25 billion to $30 billion range. But I think it's significant that the company continues to steam along while ConocoPhillips (NYSE: COP), for instance, is trimming its own spending.

Stomping on the gas
Further, Exxon expects its oil and gas production to climb by 3% - 4% this year. Indeed, the eight projects that started production last year are likely to add about 400,000 net barrels per day this year. And should the company's pending purchase of gas producer XTO (NYSE: XTO) close by midyear -- which is expected -- the increase should be more substantial.

Clearly, ExxonMobil is shifting at least a portion of its emphasis from oil to natural gas.

In fact, in his remarks, Mr. Tillerson indicated an expectation that the contribution from natural gas will increase "significantly" in the decades ahead. In that regard, the company is actually putting its money where its mouth is. For instance, it completed last week by announcing that it has finalized financing and marketing agreements for a giant liquefied natural gas project in Papua New Guinea.

Further, it is participating with Chevron and Shell (NYSE: RDS-A) in the giant Gorgon LNG project off Western Australia. And in addition, its acquisition of XTO will help Exxon toward extracting gas from shale rock, both in the U.S. and ultimately elsewhere, such as Eastern Europe.

Finding oil in the toughest of places
Looking at its oil production, Exxon is in the same boat with the other majors: It must garner progressively more of its reserves from areas that are technically challenging to reach and therefore expensive. For instance, while it's had success working with the likes of Petrobras (NYSE: PBR) in the pre-salt deepwater off Brazil, the oil and gas found there will be difficult to produce. The same is true of its demanding work on the arctic Sakhalin Island off eastern Russia, along with the tar sands of Canada's Alberta tundra.

And then there's Iraq, where Exxon was one of several companies to sign production agreements last year. As Mr. Tillerson told the analysts, the company has completed initial evaluations in the West Qurna field there. He also noted that security issues, while still present in the war-torn country, continue to decline. He also refused to discuss possible volumes from the field.

Upstream and downstream challenges
I trust you'll agree that the movement into more unconventional and challenging production sites will be anything but beneficial to ExxonMobil's -- and its peers' -- margins. Add in the issues created by squeezed refinery margins, and you have a bevy of potential difficulties for the integrated companies to attend to. In fact, Exxon's position as the world's largest global refiner is unlikely to protect it from downstream weakness for the next several years.

But despite its potential negatives, my recommendation for Fools is to maintain an energy presence in your portfolios. Given a somewhat longer-than-normal investment time horizon, I'm convinced that Exxon could provide some added value for you.

Total SA is a Motley Fool Income Investor pick. Try any of our Foolish newsletters today, free for 30 days.

Fool contributor David Lee Smith doesn't own shares in any of the companies mentioned above. He does welcome your questions or comments. The Fool owns shares of XTO Energy.  The Fool's disclosure policy operates in the most difficult of conditions.