I love the kiloHearts range - basically a collection of plugins that can run either as normal plugins in the DAW, or as “snapins” in one of their modular hosts, allowing you to easily build chains, nets, multiband rigs etc, with powerful modulation capabilities.
They recently added modular synth as well, Phase Plant, which has since become my go-to synth for from-scratch programming. It has all the usual oscillators, wavetables, samples, filters, practically unlimited modulation capabilities etc, but is still very quick and easy to use, at least if you know your audio synthesis basics.
Also, you can now recursively use the snapin hosts as snapins, basically eliminating all limitations to number of chains, number of bands etc, and this also makes it possible to build custom multiband effects and whatnot, that you can then save as presets for use within new designs.