I'll talk about those I know well :
I'm actually currently investing in synths for my own project and also because I'm getting interested in electro music... I even spent the day learning one of them! So here is a list of no-brainers :
- spectrasonics Omnisphere (ambient, pads, but it also has a lot of good analog sounds and FX). If there is only one synth to buy, it is this one, trust me ! You can do pretty much all kind of sounds, and the after sale is pretty good. There is even an iPad app to control its new X-Y interface on the touchscreen wireless, for free (if you're a legit user of course)
- native instruments Massive and FM8 for more conventional but very powerful possibilities, really really powerful synths, no-brainers as well !
- native instruments Absynth is more leaned towards organic/ambiant/dark sounds so it's not a must-have and more like another tool in your studio, but I fell in love with some of its sounds, it has a very specific sound signature. If your music has a big place for intimate sounds, you might get interested in it as well.
- Sylenth1 is one of the most impressive ones I tried because it has a very analogue-ish feel. I discovered it only recently, I'm -this- close to buy it cause it's lead sounds make my ears orgasm!
- Arturias minimoog ; I don't have this one because it's pricey just for one emulation and I'm sticking to the moog sounds from omnisphere until I can get it, but it sounds very very good, one of the best moog emulations out there so you're pretty much done with this one for moog type of sounds. Native instrument has a line of analogue emulations too iirc but I don't know them personnally.
- Camel Audio Alchemy : I don't own this one, but I'm very interested in it. It has a proprietary morphing engine, so whenever you think you open a preset, in reality if you use the XY pads you have an infinity of them. You can buy sound banks of presets on the website for more specific types of sounds as well.
There are a few other very decent options in the 100-200€ rane as well out there. In the PC world you have many freebies as well, I remember Synth1 and SUPERWAVE8 for example. They are not all-in-one solutions, you won't have more than a handful of presets on them, but they sound very good for cheap solutions. They are enough for basic synths or pads to go with your demos.
Good link as well :
http://www.musicradar.com/tuition/tech/the-20-best-vst-plug-in-synths-in-the-world-today-262145