Ok this will be a good thread.

oops meant to say...good bottle of wine and hot babe...who cares about the rest!

Speakers, unless you are using "source" correctly and referring to the records or cds that you are listening to. And then it would still be close. The simple reason is that speakers are much less accurate and have much higher levels of distortion than any electronic components, and are therefore the limiting factor. Modern amps, cdps and preamps are just not a problem. Even a little TEAC or Yamaha minisystem sounds fine if connected to a pair of good speakers. It gets a little more complicated if you're talking about record playing equipment and cartridges, because those things can be screwed up, cartridges almost as much as speakers. But, speakers vary so much in what they do, and almost all are colored and distorted in one way or another.

For those who say the listening room is even more important than the speakers, I respect that point of view, but there are some speakers that work well in real world rooms, because their designer takes real world use into account. So, I find the limiting factor in my enjoyment of music is the real source, ie, the work of the recording engineer, because my speakers work in real world rooms.

Speakers are the most important because they come first for me when building a system. I don't know the power requirements of the amp until the speakers are chosen. The source can be chosen first, but it has no limitation of working within a system like the speaker and amp combination. A particular source will work with any speaker/amp. A preamp may or may not be needed depending on the source or if an integrated amp is possible. Since all the choices can be based on subjective preference, it seems the answer comes down to other non-subjective parameters. The primary one that comes to mind is the wattage requirement of the speakers.
That is not to say I chose my speakers based on their wattage requirement. Room is a good answer too because not all speakers will work in all rooms.

The Eagle has flown