If I should get only 1 synth (some will claim its a rompler, not a synth. more on that a little down in this post) today it would be Nexus2. Uses little system resources and has tons of sounds. Many good expansion packs. A little EDM orientated but well suitable for dance, pop, house etc.
If I was to buy soft synths today I would get them in this order (based on the synths I know):
Nexus2, a good piano plugin if you are into piano playing (I use Ivory),
Omnisphere,
Komplete 8/9.
I have TONS more
plugins but rather than getting more than these I would go for sample libraries. It´s a little hard to suggest you something without knowing if you are making classical music or SHM stadium anthems.
There are really many smart and cheap expansions for
the Komplete sample player Kontakt. Kontakt is free from Native Instruments. I´m currently using 2 guitar libraries for kontakt from
Welcome to Indiginus. The acoustic and Torch. They are brilliant and cheap.
It´s really hard to tell. Don´t go over the top buying plugs. Maybe start off with Nexus2 and Kontakt and get some cheap libraries. Keep in mind that you cannot operate Nexus2 like a full-blown synth with LFOs and such. But I bet thats not your need atm. You can anyways do that with logics stock synths which are brilliant imo.
To suggest an ultimate synth list for you I would have to know more about what instruments you have/play, what music you make, what computer you have and what budget you are on.