One of my favorite soft synths is Element by Waves (now updated to Element 2). Such an authentic analog sound and so easy to get those wide, creamy pads and pristine plucks. I'm also a big fan of Rounds by Native Instruments - once you get your head around it you can make some unique sounds that are next to impossible to do with any other synth. It's also great throwing some MIDI at it and not quite knowing how it's going to interpret it.
Recently though I've fallen in love with Reaktor Blocks. That's the most fun I've had with synths in a long time.