I think you are asking the wrong question.
The most powerful blowback pistol is probably the Gamo PT-85 Tactical. It gains that power thanks to a barrel that is around ten inches long, covered by a fake suppressor. It's hardly handy. I'm not sure it's terribly accurate, but don't have relevant experience (except with the non-blowback standard-size PT80, which is ho-hum in the accuracy dept).
The most accurate and powerful blowback pistol I can think of apart from the PT-85T is the Umarex Desert Eagle. But the Umarex Desert Eagle is expensive, and plasticky. It is, like the real thing, huge, unwieldy, noisy and generally silly.
And they are both pellet firers.
Others will know better than I, but I suspect if you asked "which is the nicest blowback BB pistol?" the answer would be some variant of CZ75 or 1911 copy.
And if you asked, "which is the best CO2 repeater?", the answers would include the Crosman 600, Drulov, and Umarex CP88 and S&W 5/686.