cable connection

There might be no answer to this... but if you had  just one audio cable to upgrade , would it be between pre amp and amp or  pre amp and cd player.
Preamp and source for me. Second choice would be from the wall outlet to the amp. 
Let’s see how many different options we get on this😀
Source <> preamp
Once you lose data upstream, you can't retrieve it downstream. 

Not mentioned, but if the system has a record player, the tonearm internal wiring if it needs it. An uninterrupted run from cartridge clips to RCA plugs.
Source to pre-amp/integrated amp.

Thankfully a choice I will never have to make, having been smart enough my whole career to run integrated amps. But if I ever did go separates I would like to think I'd be smart enough to know the best cable goes between the pre and power. That way every source always goes through your best IC. The idea of once its lost upstream is a red herring. At least I think that's what its called. Do herring even swim upstream? Nevermind. The herring is a MacGuffin anyway.