The most important piece in a stereo system???

The people and home around it.

I think all of the pieces are equally important, so I think that you should start with one piece that you really know and like the sound of: (speaker,amp, pre-amp,or cd player), then build the rest of your system around it to get the sound that you are looking for. One piece of equipment in your chain can drastically change the sound you are seeking, so audition carefully around the piece that you have selected to start with, and when your finished, you will have a sound that you should truly enjoy.

The most important component is the weakest link component. Could be anything from cables(not likely but possible) to input source to speakers to your room's accustics.

One disagnostic I use is to purchase a very good set of headphones. This elliminates the room, speakers, amp, and some cables from the question. If the headphones sound much better then your speakers then you have a weak link in speakers or amp.....

Got to separate the variable.

Try think of the stereo system as a human system; which part can you say is the most important ? Can you compromise an eye or an ear and still call it a good system? I think you will then get your answer !

Preamp! Don't skimp on it. I wouldn't have thought it before I got into this hobby, but my preamp upgrades have offered the most obvious improvements in sound (Arcam Alpha to Bryston BP20 to Sonic Frontiers Line 3). To answer those who voted for front-end, I believe the 'garbage in garbage out' arguement but I've found the difference between a good $1000 player and a good $6000 player more subtle than such a price difference for other components. As for speakes, I've found speaker enhancements offer the biggest change in the character of the sound but not necessarily the quality - excellent speakers with crap electronics will merely show how bad the electronics are. My friend had B&K electronics drive a pair of Nautilus 804s and the sound was so harsh I was keen to leave the room. Overall, I've found every component matters - I've found my money best spent on a system with roughly equal expenditure on all components and cabling accounting for no less than 20% of overall system (System is YBA CD1a, SF Line 3, Blue Circle BC2s, Audio Physic VirgoII, SPM Ref interconnects and FIM Gold Speaker Cable).