Isn't Brian May's tone at least on some of the recordings attributed to "the Deacy" amp, a solid state amp built back in the day by John Deacon? It's been a while, but I think I read for practicality purposes he uses Vox amps live, but has had replica Deacy amps built for recording purposes. Still has the original, reverse engineered...in a vault, triple locked, inside another vault, hounds of hell standing guard.
Greatest being totally subjective and not necessarily relevant outside of serving the song/project at hand, +1 for Brian May's tone on many of the Queen recordings.