Sylvester McCoy is far from abysmal. I love his Chaplin-esque touches and the dark mystery he bought to the character. He could be unconvincing sometimes when called upon to shout but that's because it didn't fit his character as well as the brooding menace he bought to it.
I rate Patrick Troughton as being far and away the best actor to have played the part. He's the only Doctor actor I'd put up there with the best of the craft.