lol the best synth is not a universal thing, buuuuut there are some synths that are just more feature packed and omnisphere is one of those powersynths but i grew tired of the first one and just never updated. However 2 has useful import your own sound abilities now so i won't throw that under the bus ever.
But omnisphere shouldn't be having problems in ableton i'd look into that with spectrasonics.
I won't list alternatives solely because that is not the question.