Billionaire Warren Buffett says investors should buy stocks they understand and companies with competitive advantages that will help them grow in a limp economy.

With that in mind, here are four companies I rated "buy" in 2008 that I remain optimistic about this year. These companies operate in transparent markets and have strong financial positions, which we believe will help them outperform competitors and major indexes.

Green Mountain Coffee Roasters

(GMCR)

is benefiting from the decline of

Starbucks

(SBUX) - Get Report

. The company sells brewers to homes and offices "at cost," but profits from the "K-Cups," single-serving coffee packs, the machines use. As the economy weakens, office workers are

choosing free coffee

over $5 lattes. That shift helped boost the company's first-quarter net income 392% as its revenue jumped 55.8%. The stock has gained 12.4% in the past six months.

Ralcorp

(RAH)

is the largest U.S. producer of store-brand, or "generic," food and goods. As the recession crimps household budgets, consumers will buy cheaper brands to save cash. Ralcorp's revenue and earnings grew substantially during 2008 and the first quarter of 2009. The company added some more-profitable products with the acquisition of Post cereals from

Kraft Foods

(KFT)

last year. The company's 0.62 debt-to-equity ratio reflects its low debt, and its strong cash reserves are evident in its current ratio of 1.33, a measure of liquidity. The stock has risen 13.7% in the past year and 4% in the past six months. The shares could climb even more.

The Buckle

(BKE) - Get Report

shares are a steal at $25. This denim retailer has increased revenue, earnings and same-store sales during the worst downturn since the Great Depression. The company's same-store sales rose between 13.5% and 19.7% each month from September to January, beating every other major U.S. retailer. The company has a strong cash position, reflected by a 2.95 current ratio, and zero debt. It's trading below its 52-week high of $44.57. The Buckle is flourishing. Denim is still in. Consider this stock.

Marvel Entertainment

(MVL)

, the company traditionally known for its comic books, operates a stand-alone film studio. It has a lock on the

lucrative "superhero" genre

and released two of last year's biggest movies,

Iron Man

and

The Incredible Hulk

. Box-office hits can produce huge earnings gains for Marvel. Its comic-book business is also thriving, generating the highest margins in publishing. While Marvel shares have fallen 23.2% in the past six months and 9.8% from a year ago, its third-quarter net sales rose 47.6% and net income increased 39.6% from a year earlier. The company's next movie,

Iron Man 2

, is scheduled for release next year, but if

X-Men Origins: Wolverine

from

News Corp.'s

(NWS) - Get Report

20th Century Fox is a box-office success in May, Marvel's shares might jump on renewed optimism.

Uncertainty continues to plague the market. As financial-services stocks continue to fall, pulling the rest of the economy down, investors have become justifiably skeptical. However, stocks are selling at cheap prices and some companies are thriving despite the recession. Long-term investors should consider these companies. They have straightforward business models that are resistant to economic decline.