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

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.  
Speakers without a doubt.

Anyone can hear the difference between nearly any pair of speakers in a blind situation.

Hardly anyone, (including industry experts),

in blind tests, 

can consistently identify the difference between any decent solid state amp, solid state pre-amp, solid state source equipment.

I am not saying subtle differences don't exist, just that the audible differences are extremely minimal, perceptible perhaps, but not measurable to any extent compared to speakers.

A sound level meter is a wise investment, before you begin, learn to use it now, with your existing system, write stuff down. Do not expect even near perfection, just go for this is better than that.

Speaker differences, speaker placement differences, will readily be identified, by you and by the sound meter. Hook up a million dollar CD player, good luck getting a million dollar sound meter to find any difference.

I advise limiting your choices to speakers with high efficiency (less power needed for any listening volume, less power needed for momentary dynamic peaks, especially bass notes).

Take your own CDs with you for auditions, ones you pick to reveal speaker differences, highs, mids, bass, favorite voices, favorite instruments, ...

Next, which speakers are the best choice for your listening space? That is very hard. I strongly advise taking your time, reading, learning the fundamentals, (doing your best to ignore marketing mumbo jumbo).

I advise avoiding ports or passive radiators, go with sealed boxes. The worst situation is to end up needing acoustic treatments due to improper speaker choice and/or placement.

AVOID too much bass, helps avoid room interaction problems and keeps you in a more affordable world of choices. Ports and passive radiators are to get more bass out of a smaller enclosure. Great, IF in a space where they can work as intended, which is most often not the case.

Relocation: People love finding the ideal location and 'spiking' them. Not me. Realistically, I move my speakers from best room placement when off, forward to intermediate casual listening (still some less than ideal bass boost/room interaction), and move them into their ideal location for focused listening. No way would I limit the use of my room, by leaving them in their ideal location. Weight, and height make important differences, that is why I mention it before selecting speakers.

Efficient speakers, NOT NEEDING much amp power. That is the key to happy use of Tube amps, or lower power Solid State amps. In either case, efficient speakers put you in a lower cost need for power.

You will change everything over time, efficient speakers will be your friend for life.

At some point, stop tweaking, learn/experience more, acquire more music!