Age of Empires II: Age of Kings' soundtrack would always play in order, and this was the first song to play. I remember watching my dad play Age of Kings on his computer when I was a really little kid, and either it or Crash Bandicoot was the first video game I ever played. I actually lost my first tooth watching him do the Saladin campaign, which is ironic because this thread is me procrastinating from an essay I'm supposed to be writing on the First Crusade (although Saladin was involved in the Third Crusade, as any fule kno). Unfortunately, I don't really play AoK that much - although I do have it in my Steam library - because coming to it as an adult it just doesn't feel as magical as it did back then. I think its very surreal, slightly hypnotic soundtrack probably contributed to that feeling.
I first played Mass Effect when I was 11, and from that age to about 15 I was completely obsessed with it. I'm almost certainly biased but as far as I'm concerned no video game series has come close to it in terms of effective world-building, and I still feel a genuine love for that universe. My ideal home would still almost certainly resemble the Presidium's architecture, and would probably have this music piped through it constantly.