Rhys
Most important: Acceleration, Handling
Worth Developing: Braking, Fuel economy (gets less important the higher the stat), Downforce (least important of these, until FE gets high)
Not really worth it: Tyre economy
Useless: Cooling, Reliability
N.B I put braking there as preference, many others would have it the category above, but it's based off personal research across my accounts since the major 3D and track updates recently
I used to put braking as top priority while researching accel, but I haven't found that nearly as effective as before so I tend to agree with Rhys on this advice