What class of amplifier has the most high definition sound ?

Practically, high efficiency good quality Class D amps are capable of delivering the most high definition sound with the widest variety of speakers.

Otherwise, at the other end of the spectrum, low efficiency Class A amps can do the job very well also but you typically need larger, higher efficiency, easy load to drive, and typically more costly speakers to get the deed done.

How loud one need go is another key factor. A lot of detail listening is done practically  at modest volume, which is a lesser challenge for most amps.

Please define "high definition." I have no idea what is meant by "pure sound."

Though I haven't yet heard one (soon to be rectified), I am curious if the answer turns out to be a direct-drive tube amp driving an ESL loudspeaker. No amp output transformer, no ESL step-up transformer---sounds very promising, right? An OTL driving an ESL gets half the way there, but that designs (generally) somewhat high output impedance brings its own potential penalty.

"High definition" is very much dependent on design.  Except for the hopeless designs of Class D.  Nothing can bail out a design that is so fundamentally flawed.

I can't believe no one got that.......

Oz