Bob Sandage and Phil Farrell gave a presentation at the American Homebrewers Association (AHA) conference this year entitled "Does the Type of Fermenter Affect Your Homebrew?"
Their experiment revolved around brewing a large commercial-sized batch of Kolsch. The style was chosen because it would easily show any unexpected fermentation issues or differences, and required no post-fermentation processing that might have affected the flavor of the finished beer.
The large commercial-sized batch was divided into a variety of different fermenters used by homebrewers, including plastic buckets, carboys, cornelius (corny) kegs, and stainless steel conical fermenters. Where possible, blow-off tubes and poppet style airlocks were also used to gauge any difference these might make.
Finished beers went through a blind taste-test to determine which tasted the best. They were also subjected to a laboratory analysis.
The taste test rated the beer fermented in a carboy with a blow-off tube and the beer fermented in a stainless steel conical fermenter with a poppet airlock as the two best.
Laboratory testing rated diacetyl and pentanedione levels highest in the samples fermented (in order) in the carboy with blow-off tube, plastic bucket, corny keg, and carboy with poppet airlock. Levels were lowest in the commercial unitank fermenter and in the stainless steel conical fermenters. Levels in the smaller conical fermenters were comparable to those in the large commercial unitank fermenter.
To me, the take-away here was that you'll probably get your best results overall in a stainless steel conical fermenter. This would seem to minimize unwanted compounds while resulting in a beer that scores well with taste testers. Fortunately, I recently added an SS Brew Tech Brewmaster Bucket to my setup. This conical stainless fermenter should help improve my beers.
If a stainless steel conical fermenter is out of your price range, a glass carboy with poppet airlock is your next best bet.