Less is better:
Radeon RX 5600 XT: 172.6775
GeForce GTX 1080 Ti: 233.708
Radeon RX 5500 XT: 263.773
GeForce GTX 1070 Ti: 292.179
Be careful with two important things when choosing a card. In addition to render time also take into account the vRAM amount on each card version. And regarding OpenCL, the times that I show are the average/median times for all the scenes and all OSs. Apparently people are reporting that OpenCL does not work equally well in all scenes. I didn’t take the trouble to make comparisons for each scene individually, which is possible from the Blender benchmark filters.
Edit:
I don’t want to influence card purchase decisions just by showing render time. Currently AMD users are having a lot of problems:
https://blenderartists.org/t/blender-2-83-lts-and-amd-gpu/1233087
Also, I think that for OpenCL it is still not possible to use a precompiled Kernel, therefore it is necessary to compile Kernel every time you open the scene (one of the reasons why I would not use an AMD gpu for Cycles)