The best one to get right now is the Nvidia 1060. There are cheaper ones but it will help you the most in the long run.
Edit: The next step down would be an Amd 480
Followed by an Amd 470
PC Specs: CPU: 7800X3D || GPU: Strix 4090 || RAM: 32GB DDR5 6000 || Main SSD: WD 2TB SN850