Gordon Ramsay is a world-class celebrity chef, so it's not surprising that he always criticizes the food at the restaurants he visits in Kitchen Nightmares -- not to mention the fact that there's usually something wrong with the food anyway, or he wouldn't be there. However, I find it a bit difficult to believe that there's never anything he's ever liked in the history of the show. My thoughts are that they just don't show it on camera when he does enjoy or compliment a dish, as that would be less entertaining as his creative insults.
To back this up, I've recently visited Pantaleone's, an Italian restaurant featured in an episode aired last April. Pete, the owner, mentioned to me that Ramsay actually liked his pizza crust recipe (which is still the same), and that certainly was not revealed in the show! Does Ramsay actually like certain dishes more often than we, the audience, realize, thanks to editing?