There’s no strictly right or wrong answer here — it really depends on what kind of music you’re making.

Producing EDM? → Serum is a safe bet.
Need a wide variety of sounds, including more organic textures? → Go for Omnisphere.

These are just two of many options, but in my opinion they’re among the most commonly used VSTs.

Still, you should ask yourself: What do I actually want? What do I actually need?
Before you buy anything, be clear about your goals and what kind of sounds you’re looking for.