tonearm - cartridge - phono stage have made biggest improvements individually in that order for my system. But if I started out with an excellent source way back, it wouldn't be applicable.
So start with the best you can afford of each component, focusing on the source point through to the end point. A signal can not improve as it goes through the process. You can only get what is picked up by the stylus on the record. Unfortunately it only goes downhill from there, so you should start at the highest place possible.
And to quote a banished former contributor, "everything matters". Except tweaks that don't do anything, or worse yet, degrade the sound.