If I was told that I could only use one plugin for the rest of my life, I would choose Halion (immediately and without question).
This immensely powerful, complex and deeply misunderstood instrument has everything you need to create...whatever it is you create. The sampler is deep and its stock collection of sounds is outstanding.
The quality of life improvements in version 7 make it a bit easier to use (NO MORE DONGLE).
FM lab is immense, X-LFO is great fun, and the spectral resynythesis engine is insane (the new timestretch algorithm rivals the best in the industry). The wavetable and granular synths are still stellar, and direct sampling has vastly improved throughout the years.
Halion is not an intuitive instrument. It demands time and patience to learn. Get comfortable with the manual, because you're going to need it. Keep it close and spend time with it.
FM Lab might be the best FM soft synth ever created.
It will deeply reward those who choose explore learn it. You can define your career with it.