I've ridden all four - Thriller at the Oktoberfest; Olympia Looping at the Oktoberfest, Annakirmes & Düsseldorf's Größte Kirmes am Rhein; Mindbender at Galaxyland, Quimera at La Feria Chapultepec Magico & as Dreier Looping at the Oktoberfest. Thriller was the most intense and my favorite of the Big Four. As has been stated Quimera was more intense than I remember Dreier Looping was, although it was a good dozen years between when I rode each of them. I put Mindbender somewhere between Quimera and Dreier Looping, as far as intensity. Never been a great fan of Olympia Looping, even though I've ridden it far more often than the others.
However, my favorite Schwarzkopf looper is Teststrecke. Went to the Hamburg DOM for it's debut (many CFers could have joined Brad, Ryan, Aidan & I, but flaked out and drove past it because of the long day at Heide). Not as intense as the Big Four, but love what Meyer und Rosenzweig did with it after purchasing it from Dorney. Those SAT rides trains <3 I also go to the Stuttgart (Cannstatter) Volksfest whenever possible just to ride it.