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 60 factors.

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

Note that no provision is made for off-balance-sheet assets such as unrealized appreciation/depreciation of investments, market value of real estate or contingent liabilities that might affect book value. This could be material for some companies with large underfunded pension plans.

First up is

Manitowoc

(MTW) - Get Report

. Rated a buy since May 2005, the company makes cranes, food service equipment and marine products. Manitowoc demonstrates notable revenue growth, significant EPS improvement, impressive net income growth and stock price appreciation that has significantly outpaced its industry and the

S&P 500

.

Given the company's other strengths, its low profit margins are not a threat to its buy rating at this time.

Rated a buy since November 2005,

Northwest Natural Gas

(NWN) - Get Report

distributes and sells natural gas in Oregon and southwest Washington. Margin expansion driven by a growth in its customer base, commodity cost savings and benefits realized from restructuring activities has led to strong growth in net income. Its customer growth of 3.0% over the past 20 years has outstripped the national average by two times. Northwest Natural Gas recently raised its share repurchase program, and it offers an attractive dividend yield at its current price.

Risks include higher gas prices, which may force customers to lower consumption and adopt better conservation techniques. Any unusual changes in the weather could negatively affect future results.

Watson Wyatt

( WW), a human resources consulting company, has been rated a buy since May 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 low profit margins.

Rated a buy since May 2005,

Cummins

(CMI) - Get Report

designs, manufactures, distributes and repairs diesel and natural gas engines and electric power generation systems. The company shows steady revenue growth and positive EPS growth.

The company is aggressively pursuing a cost optimization strategy by adopting six sigma, global sourcing and technical productivity initiatives. It also plans to enter the light-duty diesel market in both the U.S. and Canada. Cummins distributed a 2-for-1 stock split on March 8.

Because Cummins operates in various competitive markets, the buy rating depends on the economic conditions of the automotive, construction and general industrial sectors. Growth prospects would likewise be dimmed by any decline in margins or return on equity.

Today concludes with

West Pharmaceutical

(WST) - Get Report

, which manufactures components and systems for injectable drug delivery and related products for the healthcare, personal care and consumer products markets. It has been rated a buy since May 2005.

The company displays revenue growth that has outpaced the industry average, sharp stock price appreciation, improved EPS and net income growth that has exceeded that of the S&P 500. These strengths outweigh its low profit margins.