Roughly speaking, PSG is the cheap synth you can hear in the chiptunes (the part producing the more or less advanced blips and blops). PCM is the format in which the samples (drums, some sound effects) are encoded. ADPCM is a sort of compressed PCM that is often used in CD games in streaming form (Sherlock Holmes for example).
IMO, two chiptune composers really stand out on the PC Engine: Kōji Hayama (Moto Roader, Schbibinman) and Daisuke Morishima (Coryoon, PC Denjin, 1943). Hayama was the first guy to use a PCM drumkit in a HuCard game, back in early 1989. At a good third place, I'll put the gents from LMS/Compile (Gunhed, Devil Crash, Cyber Dodge, Cyber Knight).
On HuCard, the best PCM showcase would be Coryoon. On CD, I'd go for Cyber City OEDO 808.