I'm glad you posted this, this is one of my all time favourite pieces of music!
I've finally got it figured out and even though my sound is only 95% accurate, I think I've come to understanding how this sound works. Allow me to explain.
I've replayed the opening of the song several times, struggling to find a good place to start, I understood it has a slow attack, long decay but I've noticed it has a clean quality to it as well, so I suspected it consisted of sine waves, and a quick zoom into an oscilloscope confirms that! I also noticed that I'm struggling to pitch the note of the synth so that means there are a few oscillators playing, detuned from each other. I can also hear pitch modulation (a VERY fast vibrato), so long story short:
3 osc(sine wave)- a few cents out of tune of each other(slow attack, long decay etc), fast(but subtle) sine wave LFOs on the pitch of the oscillators and put a reverb on the master out.
Hope my explanation was thorough and clear. I re-created this sound using sytrus, but you can just about do it in any multiple oscillator plug-in/ synth.
PS sorry for glitch at start of sample, my computer is ill :(