An interesting and important niche of pianism - left-hand virtuosity.
Its seems that actually, the one thing that defines a true super-virtuoso is the ability of their left hand - Barere, Hamelin, Libetta.
But who has the best? 
This question is impossible to answer, because the pieces for piano that involve enormous left hand virtuosity have not been recorded by so many awesome technicians.
Monsters like Cziffra, Horowitz, Argerich, Hoffman, Kissin, Berman, Fiorentino, etc. have recorded right hand killers like: Op.10no.2, Feux Follets, Op.25no.6, Gaspard, Islame, Don Juan, 6th Hungaian Rhapsody, Erlking - all pieces demanding for the right hand, but how many have recorded the Blumenfeld left hand etude? I have no doubt that Horowitz or Argerich would have played the cr*p out of it, but we'll never know...
So it is pretty impossible to answer this question...
- But I will say that Argerich's left hand octaves in the middle of Funerailles are, well, not too shabby...