Thermal Grizzly - Kryonaut Extreme - currently  best thermal paste for CPUs.

For a beast like the 9950X, I'd recommend Noctua NT-H2. It's a step up from their popular NT-H1 and performs exceptionally well with high-heat CPUs. It's also non-conductive and easy to clean off if you need to reapply later.

Thermal Grizzly Kryonaut: https://www.amazon.com/Thermal-Grizzly-Kryonaut-Grease-Paste/dp/B011F7W3LU?tag=6890-20

Any thermal paste is OK, I personally use MX-6, no issue yet.

Having used several pastes with Ryzen chips, I'd recommend either:

• Arctic MX-6: Great all-rounder, easy to apply, lasts 8+ years
• Thermal Grizzly Kryonaut: Best thermal performance but costs more
  Skip liquid metal unless you're very experienced. The regular pastes work great for the 9950X.

Noctua NT-H2.

For those pushing the 9950X to its limits, liquid metal (Thermal Grizzly Conductonaut) is still king. Dropped temps by 7-8°C compared to regular pastes. Just be careful about application since it's electrically conductive. Not recommended unless you're comfortable with the risks and need every degree of cooling performance.