best soft synths
I strongly discourage anyone claiming this for any synth.
The fact is, synthesizers like Sylenth, Massive, and honestly soon to be Serum are responsible for so many sounds because they're popular, not necessarily because they are objectively better or worse than any random synth you could think of. When you start looking at the fundamentals of sound design the actual synthesizer doesn't matter so much as what kind of synthesizer they are. Sylenth has a lot of really popular sounds already built in, so people tend to like it for that and when new producers start looking for the sounds they heard, you get people saying "oh well x y z synth is the best because a b c producer used it in all their tracks" and so they learn that synth and then when newer producers start looking for THOSE sounds they find people saying the same sorts of things.
I'd focus more on learning how subtractive synthesis works, and learning how different waveforms sound under different conditions (lots of unison and some pitch spread, 2 voices detuned slightly against each other, with one oscillator an octave higher or a fifth higher, with reverb, with distortion, with reverb and distortion (swap the order around too!), and with chorus/flanging/phasing/delay). Don't chase the gear, chase the experience.