The OPM "series" was not really a series, as it only had one chip, the YM2151....and it's series identifier....wait for it, waaaaaiiit for it: "OPM".
While there were several FM chips used in arcade games, this one was the most prolific, and probably the most powerful. This chip had limited use outside the arcade, as it was the principle chip used in the Sharp X68000, but for the most part, this was an Arcade chip.
It was an eight channel chip, each capable of up to four operators, and eight algorithm settings. It also had some special features, such as a secondary detune parameter, and an advanced LFO. It could only use sine wave operators (except for the LFO, which could use any basic waveform as well as noise), and did not have ANY sampling, or PSG capability. This wasn't a big problem though, since A) it was almost always paired with a sampler chip in the arcade, usually something from either Okidata's MSM line, or Sega's proprietary "SegaPCM" - the Sharp using an MSM, and B) it didn't really need PSG either since 1) FM can "fake a square wave" with two sine waves very convincingly, and 2) it had enough total channels that it could spare some to fake PSG, and still have enough for more serious FM applications.
Here's an example of the OPM - Shinobi (Arcade) Soundtrack:
Shinobi PCB had OPM and a sampler chip, but the sampler was used for voice clips and sound effects. As for the music: 100% pure unadulterated OPM FM!
And here are a few other examples for you (a mix of complete soundtracks and stand alone tracks) - all using the YM2151 OPM from either the Arcade, or from the Sharp X68000 (some also use PCM sampling for percussion, and some are pure FM). If you have any others you'd like to share, feel free to post away in the comments!
Akumajou Dracula (aka Castlevania) - X68000 OST - Load BGM
Area 88 (aka UN Squadron) - Arcade - Complete OST
Marble Madness - Arcade - Full OST
and lastly - Shadowland - Arcade - Full OST