What’s the best amplifier design if I want the highest fidelity?

Class-A, nothing else sounds as good and it is the least compromised design in terms of audio performance.

That being said, class-A amplifiers are hot, heavy and expensive. They also often lack power, because even low-powered class-A amplifiers are heavy and expensive to make.

High-powered amplifiers like my old Krell KSA-150 are crazy-heavy and expensive. But, when sound quality is priority number one and you have the free space to site a large, expensive, heat-generating amplifier, class-A is always the way.

Keep in mind that dynamics and the ability to generate realistic sound pressure levels are as important as smoothness and low distortion in hi-fi terms. By these measures, some class-A amplifiers will be wanting with less sensitive speakers, simply because they lack power. But get the power/sensitivity match right and class-A is the gold standard.

Also, keep in mind you can get nearly all the way there, and get better dynamic fidelity with high-bias class-AB designs, like my 500 Watt per channel Perreaux 5150B. It has roughly 30 Watts per channel class-A power. For most normal listening, this is a 30 Watt class-A amplifier, with the dynamic capabilities of a 500 Watt amp. Cool huh!