This may not be what you’re going for, but my go-to samples tend to be impulse responses. They can pretty much transform any sound into something new and inspiring. I have impulses for EQ curves, cabinets, microphones, amps, reverbs, natural sounds, and a ton of other wild effects. Paradoxically, it’s the fact that they don’t have any knobs to turn that makes them so appealing to me. I could start with a simple looping saw sample (of a single wavelength), and just with impulses, AHDSR, eq, and filters make something completely unique and ridiculous.
If you’re into designing your own sounds (or ubertweaking existing samples), impulses are amazing, and Renoise’s Convolver does a standup job.
Check the video on this page if you haven’t already gone down the IR rabbit hole (it’s madness):
As an aside: If there’s some effect plugin that I really love, I’ll make an IR out of it. The only non-native plugin I run is a VU meter, which I would totally replace with a tool if I could figure out how to code it… it’s on my todo list.
Vanilla Renoise + tools is an ungodly machine.
Some decent free starting IRs (basic, but solid):