Arcade would win by a longshot for me, considering my love of fighting game music and oldschool shooting game music (this is also taking the massive nature of some releases into account...I'm not counting stuff like my System 16 Collections or Namco Graffitis). In the lifespan of my collection outside of arcades, PS1 would probably win followed by SNES, but I've probably sold off more PS1 soundtracks as well (and most of the SNES soundtracks I have sold, I want back.
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Interesting topic.
Saturn would probably be last, I've only had a couple Saturn soundtracks. Same for the PC Engine/Duo.