The best buffer... or at least my favorite!

[Steben]

If you're that kind of guy (girl?), you can deduct "behaviors" from devices' datasheets. Some of the mistery is right there in the graphs that correlate device Hfe vs. Ic. (In short; because the Ic changes with the signal, everything else that's affected by changes in Ic also changes).

In short, and simplified, the actual device nonlinearities will effect everything that's plugged after the buffer - and before the buffer because modulated Hfe means that actual buffers' input impedance also changes with the signal.

If the buffer has it the magic "mojo" (just the right nonlinearities), your rig will sound "better" ... and vice versa.

If you seek total linearity, use a FET opamp.

If you seek the mojo, use a BJT. (I mean the one that "plays" with you, not against you).

One of the most simple as linear as possible single discrete devices is ...... a class A triode stage in a large region of the transfer curve.
Classic Fender "clean" tube amps are great because they have less distortion than simple transistor amps. And many clipping elements of many tube amps are pretty hard clipping.