Absynth is well known for making good pads, and there's a couple of okish free synths that could be good starting points, Sun-Ra and Crystal, I usually use samples though.
Edit: also the fx chain you use is very important, and if you want extra stereo width use 2 copies of the same pad with slightly different settings, 1 panned hard left, 1 hard right. You want lots of chorus, phasers, reverb, flangers etc.