Snape by a long shot. Lets see... Unbridge got wasted by Centaurs. She proved immensiely weaker than Flitwick, by not being able to remove the swamp where as Flitwick removed it in an instant. Snape is clearly stronger than Flitwick. He is superior to most of the Death Eaters (In HBP he ordered them around at Hogwarts). He tricked Dumbledore. He toyed with HBP Harry, and by HBP Harry's pretty strong. Snape is stronger than Bellatrix as she got hurt by Harry in OOTP and in HBP Harry is even stronger but was no where close to harming Snape. Bellatrix beat Kingsley, who took out 2 Death Eaters at once. To suggest that Umbridge is stronger than 2 Death eaters combined is riduculous. I doubt she is even as strong as one Death Eater.