Which decaffeination method produces better tasting coffees? It is difficult to say for certain for two reasons. First, it is virtually impossible to turn up the identical coffee decaffeinated by a range of different methods, and the quality of the original coffee obviously influences the quality of the final cup. Second, decaffeinated coffees are difficult to roast properly, and subtle differences in decaffeination method may be overwhelmed by differences in the quality of the roast.
Nevertheless, my own experience clearly and consistently indicates that the Swiss Water Process tends to emphasize body, de-emphasize acidity and high notes, and occasionally (but not always) alter or blur flavor, whereas the European or solvent method tends to preserve acidity, nuance, and high notes, but may reduce body and dimension. As for coffees processed using the CO2 method, I have tasted some excellent samples, but not enough of them to generalize.