Simple, class-A, whether tube or transistor, it’s the gold standard.
That’s assuming you choose a version with enough power to handle your room and speaker sensitivity of course. This is an important consideration not to be overlooked.
Nothing sounds better than well-designed, well-built class-A amplifiers because it’s the least compromised topology in terms of sonic potential. This is why you will always find class-A designs where cost is no object, and always in low-power circuits like preamps and headphone amplifiers, where class-A can be implemented without great cost.
We can refine this even further by stating that the very best solid-state class-A amplifiers are generally MOSFET types, for various reasons. FETs behave a little more like tubes than bipolar devices do and they simplify the circuit design and number of parts needed. For this reason, cost-no-object class-A designs like the Accuphase A-75 and A-300 for example, and all the Accuphase class-A designs, use MOSFETs.
That being said, class-A speaker amplifiers are hot, heavy and very expensive, because of their need to output significant power. For this reason, they often lack power, because of the need to make them at least somewhat affordable. Even low-powered class-A amplifiers are heavy and much more expensive to make than equivalently specified class-AB gear.
High-powered class-A amplifiers like my old Krell KSA-150 or the to-die-for Accuphase A-100 monoblocks 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.
Note that an amplifier being class-A does not guarantee that it will sound great. The plethora of cheap Chinese class-A designs out there bears testament to this. You really do get what you pay for, so there are many good reasons why an Accuphase A-75 class-A stereo power amplifier costs $35,000 AUD and a Vincent doesn’t!
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 may struggle with less sensitive speakers, simply because they lack power. Get the power/sensitivity match right though and you will be winning.
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 or my new 200 Watt per channel Accuphase P-360. Both offer a smaller amount of class-A power – 30 Watts for the Perreaux and around 10 Watts for the Accuphase. For most normal listening, these are class-A amplifiers, but with the punch of something much bigger.