1) absynth granular sampler - mostly for "higher" pads, load any sample, like, part of your previous track and have fun
like this
2) any fast synth arp blurred completely with all kind of delays, reverse delays, pitch shifters and reverbs - for more "organic" pads (for example if we'll make random apr with sine sound and then add long reverb with 100% wet we'll get CBL-alike gurgling textures)
3) any "classic" synth pad with some modulations and a lot of reverb and eq
4) very rich transient drone sounds can be made with zynaddsubfx vst, if you use windows os
