Which is the most important part of a stereo system?

The single most influential component for playback sound quality is universally accepted as the loudspeakers. It’s been this way for decades, perhaps even from the days of the birth of audio.

No two pairs sound the quite the same, sometimes nothing like it.

That in no way is to deny that individual mood at the time of listening is not absolutely crucial to the received perception.

We all know just how good the human imagination can be. In the right mood (and company) I can easily imagine that my Sony television set sounds better than most of the systems I have heard.

Same thing can happen with the playback system in a club or a party, but only if I’m having a really good time.

Later on it sometimes becomes a puzzle as to how I could have believed such obvious nonsense.

The mind and all it’s tricks seem to be geared towards promoting survival and seeking pleasure, and not always towards discerning any form of truth.

Just in case nobody mentioned it already which they probably didn’t - the fuse, be it ever so humble. Another one nobody probably mentioned is electrical connections of all non-audio outlets. And one more nobody mentioned  - how many CDs are in the room.

Well, as often, valuable points of view. I'll go counter-culture. Without excellent power amp nothing will sound right with any source, preamp, speakers or cables. After that turntable itself, not tonearm or cartridge, and speakers - any order. Modest but basically good speakers will sing with great power amp, the other way around won't get you anywhere.

Another vote for the room.
It IS the most important single component affecting the sound by at least half or more of what you will hear.
It's also the most overlooked/disregarded.
The majority of us just keep focusing on the gear.
There's a reason someone came up with "audiofool"...

The room is the most important part. I’ve had a nice setup in a bad room and it sounded really bad. I would rather listen to a mediocre system in a great room over the highest of high end systems in a terrible room any day. The only exception I can think of might be dipole speakers which do a good job of “removing” the room from the equation.