Which cable is most important to spend money on...

it depends on a few different issues.

where are you now?

if you are at an entry level point and don't already have some fairly good cables then i would start with more quality on power cords (and even power conditioning) as those areas are the areas of biggest shortfall with modestly priced gear and inexpensive interconnects and speaker cables can sound pretty good and not be a severe limitation.

if you already have some quality cables then it gets more complicated. what is your priority (or sonic shortfall)? as your system and room climbs the stairway of higher resolution and coherence it becomes a case of fine tuning instead of fixing problems.

what level of bass performance (how deep will it go and how does your room support it)? if your system does plumb the depths then the priority might be speaker cables. i have found that unfortunately the very best speaker cables handle bass considerably better that the 'very good' speaker cables.....and when your system can 'go there' that last bit of control and articulation in the bass will yield amazing performance gains overall.

at the top level of system performance power cables generally come next as they can significantly influence overall system energy and the 'toe-tapping' factor.

interconnects are generally last at the higher levels of system performance as they sound much more alike as you go to higher levels of performance and price (not that they sound the same just less different than speaker cables or power cords).

i have done maybe 30 whole system cable comparisons and as my system improved the above heirarchy was quite consistent.

get the bass right with the best speaker cables you can afford, get the system energy right with power cords and power conditioning, and then fine tune with interconnects.

i can live with imperfect interconnects; but the speaker cables gotta be right.

note added; i agree with Thomas above about the significance of the tonearm/phono cable. since that cable is dealing with such a tiny signal it will have more significance than any other interconnect and is really a part of your phono stage.

regarding the order of significance of interconnects; there is not one answer. it will depend on a number of factors.

in my case my preamp (the darTZeel NHB-18NS) uses a propriatary BNC 50ohm cable (Zeel) that outperforms any conventional cable when used with the darTZeel amp. i need an 8 meter set of cables between my pre and amps so optimizing that particular interconnect is most significant in my system.

some sources have 'funny' output impedence or need long lengths. digital cables can be very sensitive to lengths. my digital uses 3 ST Optical cables which can be as long as you like with no difference in performance.

the answer here is that in a very general sense i would say that the order of interconnect significance would be;

1. tt to phono stage
2. source to pre
3. pre to amp

but at the highest levels of performance generalizing is not recommended.

If you want to catch the upgrade bug then go buy yourself a set of decent ICs and you will be in total shock with that first jump from the stock cables that come with components.

This, assuming you have components that can handle it......

Ye ole chicken v egg question, oh well, what the hell..

In order to answer your question I'm going to make two assumptions, just because you ask the question in the first place. 1) You are relatively new to audio and have yet to develope audio listening skills which will make auditioning and selection a simple task, and 2)that you are just developing your system and have not yet acquired highly refined equipment.

Regarding power cords - These are highly equipment dependent and the benefits of the different cords can be very subtle. To begin with I would get some inexpensive heavy guage Belden cord to use as a benchmark.

Power conditioners - more important than cords by a mile. Highly recommended if for no other reason to obtain something (filters) to seperate your CDP from your amp/preamp.

IC's - as in power cords the difference between good ones is subtle but there are some which are not subtle and can be used to really act as tone controls (not recommended). As with the P/C's I'd start off with an inexpensive IC noted for neutrality and use it for a bench mark.

Lastly there is speaker cable. This IMHO can produce the most easily audible differences to the tyro, especially as the lengths get longer. I would start off with an excellent low cost heavy guage cable to use as a benchmark which will reveal changes brought by other stuff as you trade up and fine tune your system. I highly recommend Canare 4S11 Star Quad for this purpose.

For a specific answer to your question, starting out I would get a good power conditioner, with a filter for CD power, and speaker cable. Next I would work on IC's and lastly PC's and then revisit speaker cable.

In the final analysis though all of the wiring is important to getting the best results from you system, however this should be consistent with your ability to detect the differences. There is no reason to buy stuff until you can hear the differences. This ability usually only comes with a lot of experience and very careful analysis of the sound of your system.

As in everything wire related, IMHO and YMMV. :-)

>>He says they're the best<<

Everything is system dependent. There is no universal "best" cable.