Each weekday, TheStreet.com Ratings compiles a list of the top five stocks in five categories -- fast-growth, all-around value, large-cap, mid-cap and small-cap -- and publishes these lists in the

Ratings section of our Web site. This list, updated daily, is based on data from the close of the previous trading session.

Today, mid-cap stocks are in the spotlight. These are stocks of companies that have market capitalizations of between $500 million and $10 billion that rank near the top of all stocks rated by our proprietary quantitative model, which looks at more than 62 factors.

In addition, the stocks must be followed by at least one financial analyst who posts estimates on the Institutional Brokers' Estimate System. The stocks are ordered by their potential to appreciate.

Today's list begins with oil and gas transportation company

Oneok Partners

(OKS)

, which has maintained a buy rating since June 2006. The company has recently acquired valuable natural gas and processing assets that reduce its dependence on regulated fee-based transportation revenue.

Oneok shows impressive revenue growth and return on equity that is significantly higher than the industry average. It also has enjoyed notable net income growth.

Risks to the buy rating include an unexpected rise in interest rates, which could drag down profits because the company has taken on more debt to fund its aggressive expansion strategy. Oneok is also vulnerable to the risk of a compressed gathering and processing margin, which is related to volatile natural gas prices.

Watson Wyatt

( WW), a human resources consulting company, has been rated a buy since March 2005. The company's strengths include steady revenue growth, solid stock price performance, impressive increases in net income and a compelling record of EPS growth.

These strengths outweigh the company's weak operating cash flow.

A buy since March 2005,

Baldor Electric

( BEZ) designs, manufactures and sells electric motors and drives, and other parts. Its custom and stock products are sold directly to original equipment manufacturers, as well as to independent distributors for resale. The company's strengths can be seen in its stock price performance, revenue growth and improved return on equity.

Risks to the buy rating include higher material costs and any slowdown in the economy, as demand for its products depends on the levels of industrial activity and capital investment.

Carrying a buy rating since March 2005,

Martin Marietta Materials

(MLM) - Get Report

provides construction material for infrastructural, commercial and residential uses. The company displays solid revenue growth, an impressive pattern of EPS growth over the past two years, compelling net income growth and increased net operating cash flow.

Terex

(TEX) - Get Report

, rated a buy since May 2006, makes equipment used for construction, mining, shipping, transportation and numerous other industries. The company has shown impressive revenue growth, a pattern of solid EPS growth, compelling net income growth, and outstanding stock price appreciation.

These strengths outweigh the company's low profit margins.