AYM because I got an Atari ST when I was 10 and it became my first composition tool, using Quartet (which can be converted to .MOD if anybody wants to try something stupid) and Protracker. Tried programming the YM with Sidsound Designer but it was too difficult to handle without any knowledge of English or music. In my days .MOD were not called chip music and botb would've awarded mixist points, not chipist. I mean, it's just fucking samples! </rant>
So, I got into YM to take revenge on my teen years, I needed to use that magnificent sound.
2nd reason, the YM is way deeper than the engineers designed it to be with its 3 square channels and the noise generator: the envelope used as audio oscillator with its approximate tuning, the ST-specific faux SID sounds, the 4-bits sample playback, there is a lot of room for experimentation with this chip and I'm still not bored of it :)
That being said, my favourite chip to listen to on botb is probably the 2A03, so nsf is definitively a fave too, there is so much fantastic music on this chip, with our without expansions, and its triangle wins everybody's