I honestly don't understand this mentality. Only get what you need. Just because you have the 10 best synths, it does not mean you're going to make great music. I had to uninstall Omnisphere because quite frankly, I was feeling confident with other synths I possessed. Layering really helps ANY sound gain depth, movement, warmth and whatever else. My go to synth till this day is Novation V-Station and Adventus 1.5 - Doesn't matter how old these VST synths are, I'll always have it in my collection cause they do what I like them to do.
Yes, Serum blew my mind and I started using it for Bass. I also like Sylenth1 and Absynth5 because it's so modifiable.
Then I have Dune, Diva, Ace,
Z3ta+2 which I tend to use a lot. The Arturia stuff for modular, classic synths like and/or mids & highs,
Korg M1 + MS20, Elek7ro, Massive & FM8 NI is awesome, but I skipped on Kontakt, but I might ditch some VST and get ver. 4.
Rob Papen's Predator and Albino are also wonderful. Discovery Pro I use a lot. And you REALLY can't forget G-Force's ImpOSCar & The Oddity. So I never set out for a top 10 or 20 list. When you run into a speed bump, or you just find something new that has a few features you'd really like to have, THEN get it. But don't get something before you even know what's in it.