The Original Saleae logic analyzer (or one of its clones -> search for “Compatible Saleae”) or a LA104 (maybe with custom firmware) or maybe a DSlogic pro or ChronoVu. You can also browse the HackADay Archive to find an analyzer that suits your needs (e.g. build your own, or based on a pico). It all depends on the resolution/speed you need and usecase.
USB isolator, $10 saleae clone and sigrok/pulseview setup is very simple and cheap (except for the isolator).