Poll: Most impactful component

The answer depends on the system.

My system is balanced and optimized, with all components compatible and at the same level. So, there is only one answer, upgrade my speakers from Sonus Faber Amati traditional to Sonus Faber Lillium. While the Lillium are a bit big for my relatively large space and would cause bass loading at high volumes… I don’t listen at high volumes. So this would make a big difference. Other component upgrades not so much. 

Of surprise to me, power amp has consistently been the biggest difference maker in my journey.  

My room. Whilst I'm grateful ful I have a dedicated room, I know it is too small for what I want to achieve, so I have to live with compromise despite room treatments

Right now my source is the weakest component. But my room is not great either.

Duh, speaker, of course. Everything in the chain matters. But speakers vary significantly more than any other link in the chain(the cartridge is second in an analog system) so their choice gives many more variations to find sound you like and conversely it is by far the link in the chain that can produce sound you don't like and there are probably many more speakers whose voice you don't like than you do like.