Complex oscillators...OK, they begin with Don Buchla and his dual waveform generators for the Buchla 100 system, and gradually evolved into something like this: https://www.modulargrid.net/u/buchla-model-259 But Buchla is hella expensive, so of course there's complex oscillators in Eurorack that get you there for cheaper. And they're very useful, because you can generate a lot of FM or AM timbres in a pairing like that...or you can use the two sections as individual VCOs, plus a whole lot of other craziness (complex oscillators as LFOs are pure madness!). The upshot is, if you've got a build that has space for them, they're primo...you really only need ONE per voice, actually, as they've got a well-deserved rep for timbral complexity that goes beyond all but the better digital oscillators.