wow thats a really good deal for any of those, reaktor is a beast, lots of options there from already made synths and modules, to making your own creations.
absynth for pads, ambience, and textures
massive is cool, pretty intuitive to program, wide array of sounds
fm8 for techno and trance type stuff
all cpu heavy for sure, too bad kontakt isn't up there!
Well, Reaktor is a programmable instrument similar in some ways to a modular. The user community is HUGE, with lots of pre-built instruments. That said, if really geeky technical stuff is not your cup of tea, I'd have to go with Massive or FM8. Massive is the most diverse sounding of them all, with a fairly intuitive user interface. You can go to Massive for all kinds of different sounds and be pleased.