Here is the curated list of the best codes based on community votes, visibility, and rarity. To use these, simply copy the code into the hex input box when customizing your rims/tires.
According to the Car Parking Multiplayer Indonesia Facebook group (the largest community for the game), the best combination right now is:
This combo is visible from across the map and makes your car look like a high-end racing prototype.
For players who want a stealth, military look.