what is more important ?

Hiya ,

Just wanted some views/opinions on what is the most important piece of equipment in the reproduction of sound/music.

I know and have heard that the dimensions of the room has the biggest impact but what about the speakers, amplification, and source... which one would give the biggest sonic differences(for better or worst) if replaced ?
If my current system needs upgrading which part of the chain should I focus on first to get the best 'bang for my bucks' ?

Cheers all

Cables make very minor differences, and that's what actual measurements reveal. 

This hobby gets most expensive and frustrating when legitimate analysis of what you're hearing isn't taking place, but rather, going to forums and pitching questions about what equipment will do what to achieve your goal, or just blindly buying gear with no grasp on what it does on a basic technical level. 

I recommend starting small and simple and learning for yourself. Nobody can actually explain to you what different distortion sounds like, and that's what distinguishes one system from another. 
Yes but they are close to last on the list.  
Want to guess what the most important piece of equipment is in a car? The nut behind the wheel.

Same here. If you know what you're doing you can build a budget system that sounds just wonderful. But if you don't, then no components under the sun will ever get you there.
Speakers. The difference between cheap speakers and good ones is huge. The difference between a cheap anything else and a good one not so much except maybe cartridge. The speakers will determine the sound of your system more than anything else. With good ones you will be able to appreciate upgrades in other components. 
Georgehifi, humidity does not effect the efficiency of ESLs unless you have adjustable bias and are forced to turn it down. Most of us adjust the bias to the highest point satisfactory for all conditions as bouncing your bias around screws up settings with other devices like subwoofers.
Like the guys said above....any system needs to start with your speakers.  Amplifier choice will flow from it and the overall coloration of speaker and amp together will help with choice of DAC, phono stage, cartridge, etc...

Exactly how much you allocate to each part of your system is TBD. I have met folks that will tell you about 50% of your system should be dedicated to speakers.  I has in a retailer in Westchester County New York and the POV was that you should maximize amplification because it will extract every scrap of ability from the speaker.  

I am in the earlier camp and am obviously biased in favor of speakers. And based on personal experience, starting with an incredible pair of speakers, I have varied between a $3000 Class D up to $17000 mono-blocks and I feel I lose less stepping down in terms of amplifier than a switch in speaker.