“With NVIDIA RTX, core ray-tracing operations are now hardware accelerated by the GPU, making this the fastest version of Cycles yet.” - Brecht Van Lommel said.
Cycles core computing method is ray trace. And if this thing is supported by GPU hardware directly, then it will be fast to compute ray trace part. So, RT core number will benefit Cycles most in general.
But the whole Blender Cycles use OSL/GLSL for rendering. Not every thing in those complex shader node bottle-neck is to compute ray trace result. See this chart in official Blender Optix support post:
You can see that not every scene got the same boost from using Optix. Since not every thing is boost by new architecture.
Also, there are some special unit to do other calculation in RTX family as well(texture, Render Output):
Those unit (including RT unit) performance are almost directly proportional to GPU Cores in RTX family. So I think don't bother on what core are best for Cycles. Cycles benefit all performance from them.