A while ago, while looking for free audio/midi notation software or something at a low price, I found this Mulab.
It seems excellent to me; it has good synthesizers, all with the possibility of modifying the waveform (which is no small thing); use of VST and ReWire mode.
Currently, I am trying to familiarize myself a bit with the sounds (because usually some sounds can stimulate ideas), but I haven't yet delved into the various functions.
Anyway, in terms of sound quality, it holds its own.
It seems quite simple to use and the free version has no substantial limitations, one of which is being able to load a maximum of 6 synthesizer modules.