I think that each great conductors and they are MANY, must be judged in some precise work from some composer...One by one...
Then there is no greatest...
But if you torture me to name one i will side with Valery Gergiev opinion and Ernest Ansermet opinion two great maestros in their own way ...
Furtwangler never directed symphonies with an EXTERNAL beat but with an INTERNAL pulse emerging spontaneously with the music itself... Valery Gergiev explain it well...
«Valery Gergiev: The most difficult thing in conducting is to avoid a mechanical beat. This endless search for a true tempo, the right tempo for every bar of music, and not just a single tempo for the whole movement, is something few conductors ever master. Few leaders will recognize, perhaps, that it is something difficult for them, but they will try to do it and compete with Furtwängler, and most likely they will fail.»
This also explain why some improvising jazz musicians are so great, or like the african youruba talking drummers, or the singing of the pygmies, they are not handicapped by the burden of INTERPRETATING with the right beat and an EXTEERNAL timing written music and more easily access to this spontaneous INTERNAL pulsating beat which emerge in each parts of the improvised unwritten music ...
it is the reason why TIME ITSELF OBEY MUSIC AND MUSIC DONT OBEY TIME BUT COMMAND IT ...It is evident with Furtwangler more than with most other maestros in written music...His interpretation is more like in jazz an irrepressible spontaneous SURGE...
By the way more than pure technical abilities it is the same for piano playing, you cannot master Lizst and Scriabin without mastering the timing flow of the music parts...it must be an internal timing coming from the music itself... most people compare pianist for their CLEAN sound or pyrotechnical ability, but it is the art of time coming from the music itself the supreme rule...
The fact that music command time is a mathematical discovery coming from Alain Connes works in non commutative geometry and his interpretation of the Zeta function related to quantum mechanic hilbert space operators algebras...very important and astounding work describing a source of variation deeper than time itself ... Music is example of this...
Listen to this interpretation where music command time , this pianist play like Furtwangler direct, none of them impose a tempo on the music but let the tempo surging from the music itself :
If you have doubts about this pianist look what Schoenberg himself wrote and contradict him if you dare...