Re: it was the music
+1 for the music. Sure the ST had midi built in, but the Amiga 4 channel stereo was head and shoulders above any other home computer at the time.
I think I spent more time using SoundTracker / NoiseTracker / MED etc. than playing games on my old A500. I managed to amass disks and disks of ripped Module files - memory could be scanned after a soft reset and a Module Ripper could save out the data - all to get at those precious samples for my own (terrible) tunes...
As for my favourite tunes, too many to list but include: Shadow of the Beast; Obliterator; Gods (amazing); Xenon 2 (naturally); Jesus on Es mega-demo; Pinball Dreams.