Most of the MU series were backward compatible with the newer ones mostly adding more voices, polyphony, samples, effects and expansion options. Thanks to MAME we don't have to choose as we can have them all including 1999's MU128 which could play 64 different voices at once with 128 track polyphony.
Yamaha's many various OPL chips were based on cost and feature reduced versions of the FM synthesis engine in Yamaha's professional standalone music synthesizers. These chips powered not only PC sound cards but also a huge variety of 80s and 90s arcade machines, game consoles and home computers (https://en.wikipedia.org/wiki/Yamaha_OPL).
It's fascinating to explore the evolution of the OPL technology as this "lower cost, lower end" alternative to pro gear gained in sophistication and power culminating in 1994's OPL4 chip (aka YMF278) and its even more capable cousin the YMF292 used in the Sega Saturn console. These chips had impressive synthesis power for their consumer console/sound card price point. By the late 90s onboard music synthesis in sound cards, consoles and arcade machines began dying due to increasing storage and CPU power permitting playback and mixing of fully sampled music tracks. By the early 2000s the only remaining consumer application of Yamaha's powerful onboard synthesis chips was for generating ringtones in mobile phones. It's kind of sad such a rich musical legacy ended up playing 5 second mono tracks on half-inch speakers (although those mobile phone chips probably made more money for Yamaha than all their pro music gear til that point combined).
Here are two good links from my retro research bookmarks: The best rundown on the genealogy and capabilities of Yamaha's FM sound chips I've seen: https://gist.github.com/bryc/e85315f758ff3eced19d2d4fdeef01c.... And a collection of very impressive OPL4 music tracks: https://www.youtube.com/watch?v=zqD52xioky4.