It depends on the sound you are looking for.
Do you need realistic sounds (acoustic instruments or voices, ethnic instruments)?
Reproductions (with all the usual limitations) of analog synthesizers?
The list is long and almost everything produced to date runs mostly on Windows (unfortunately, I say this because it is a system that often has stability problems), then on Mac, and finally on open source operating systems (Fedora, Linux, etc.).
So in a certain sense, there is no best synthesizer on Windows, but rather the (presumed) best piano developed with physical modeling, or sampled, just as there are sound libraries usable with "third-party" software, meaning not produced by the companies that developed the sounds.
One must therefore distinguish the various software into:
Sample players (single sounds or multi-sampled, soundfont, wav, aiff, etc.)
VST standalone and non-standalone (sounds in proprietary format and non-proprietary converted into the latter).
VSTs that utilize physical modeling.
Sound libraries (of every type).
This is a topic that cannot be exhausted in a few lines.