Which GPU should I pair with an i9-9900K and 600W power supply?

A used RTX 4070 is probably the best choice from that list. It offers much stronger performance than the 5060-class cards and should be a reasonable match for a 600W power supply, assuming the unit is in good condition. Just remember that the i9-9900K could become the limiting factor in some games, especially at lower resolutions.

The RTX 4070 is the safest all-around recommendation here, but check your exact power connections and the age of the EVGA 600 BR. It is a budget power supply, so I would avoid significantly more power-hungry cards unless you also upgrade the PSU. At 1440p, the 9900K should still pair reasonably well with a 4070, while at 1080p you may run into CPU limits more often.

I’d skip an 8GB RTX 5060 in 2026. A Radeon RX 9060 XT with 16GB would likely provide better long-term value if you can find one around $450, particularly if ray tracing and Nvidia-specific features are not important to you. The extra VRAM should also age better than an 8GB card.