If you're in the market for a variety of instruments from one plugin and you are looking for a lot of acoustic sounds like piano, violin, and trumpet, I'd look into getting Kontakt. It has a really nice factory library to start you out, and its the industry standard for sampling, so you can find tons and tons of add on instruments and libraries everywhere.
As for the electro stuff you may want to look into one of the synths mentioned by people above. Every one of those has its own advantage and none is a bad choice, just look at each one and find what you like the most.