This is the best site I can suggest you check out : http://www.kvraudio.com/
It's hard to suggest which one would be best for what you want to achieve, but I suggest NI Massive, ReFx Nexus2, Sylenth1, and NI FM8. There are hundreds other ones out there, but those I find to be a good starting point if you're not the best at programing your own sounds with plugins as they come with a whole lot of presets, and there are a huge number of presets available to purchase / download for free for them all.
If you want to do orchestral stuff, the best of the best would be EWQL products, but they are very expensive and take up a lot of RAM & hard drive space, so unless you have a deep wallet, I would go with Edirol Orchestral, as it gives you any instrument that would be in your standard orchestra, and doesn't use up much of anything to run. Not the best quality, but it works well and passes the quality test in my books.