I love Reaktor 6. I use the player but the sound you can buy and being able to play synthesisers with your computer keyboard is amazing. It also works fantatiscally in Live.
The use of blocks as template for making your own modular synth is really good, endless amounts of sounds to be made from it. Easy to use and quick to learn if you're a beginner. If you don't know how to build your own patch then you can use some of the pre-build patches that are on offer.