What GPU should i buy to upgrade my system?

What should i buy to upgrade my system? i have about $450-500. I'm mainly thinking about upgrading my GPU, but unsure which one?

My current setup:

CPU - Intel Core i7 9700K @ 3.60GHz

RAM - 16,0GB Dual-Channel DDR4 @ 1337MHz

Motherboard - ROG STRIX Z390-F GAMING (LGA1151)

GPU - NVIDIA GeForce GTX 1080