Over the years, stock analysts have gotten a reputation for shying away from sell recommendations. Last year, analysts who followed alternative-energy companies were willing to use that four-letter word and did well.

 “I’m not exactly shy about putting a sell rating on,” says Michael Judd, 47 years old, of Greenwich Consultants LLC, the top-ranked analyst in the sector. There is no fancy office or sales staff at Greenwich Consultants: The firm consists just of Mr. Judd, working out of his home in Rumson, N.J.

Known for his coverage of the chemical industry, the sector in which he has been the top Best on the Street analyst for three years in a row, Mr. Judd expanded his coverage in recent years to include ethanol makers and a solar-panel maker.

His style is to change his recommendations often, updating his earnings model frequently. He switched from buy to hold to sell on VeraSun Energy Corp. within the span of a week last April, and during the course of the year he changed ratings on the stock 42 times.

Investors could have done well to follow his emails closely. The ethanol maker’s stock returned about 15% during the brief periods he had buy ratings and fell more than 80% when he had sell ratings. In late October, Sioux Falls, S.D.-based VeraSun filed for bankruptcy protection, caught between high corn prices and falling gasoline prices, which dragged down ethanol prices.