I'll chime in here and second what both the guys have said already. Personally, my main go-to synths are the original Alchemy, Serum, Omnisphere and Padshop Pro..there are others, but these 4 tend to get used on almost everything I do.
by the way, if you're seriously thinking of getting Serum, then do yourself a favor and buy it through the [Only registered and activated users can see links. Click here to register]
website, as they offer a Rent-to-Own plan, where you pay just $9.99 per month. Once you've paid the full price, it's 100% yours..but in the meantime you get to use it as if it were already yours.
Also, if you want to stop payments for a while, they let you cancel and restart whenever you like and continue from where you left off.
It's also worth nothing they don't charge you any interest or extra charge for going with the Rent-to-Own plan..basically if you buy it outright or go with the monthly plan, it will still cost you exactly the same in the end..there's no hidden fees.