I get that question often and it depends.
It's all about taste,your genre and what these manufacturers bring out and update
from time to time.
Few great examples* :
Native Instruments (Komplete Kontrol) Native offers an enourmous starter package for free!
Arturia (Pigments and Analog Labs 4,VX 7),
Lennar Digital (Sylenth),
First of all,you need to start somewhere.
My tip is.....subscribe to these companies mailinglist and socials and keep your eye on
the discounts and offers. (and freebies!!!)
The offers mostly come around the holidays and can spare you a lot of money.
Also if there is a DEMO version available,download and check it out and try to find
out how it works,if it fit in your work flow and doesn t get buggy in your DAW.
Marketing this way is core for these companies and you can benefit from the discounts they offer.
(*we don't get paid by this manufacturers!)