Which cable do you think is most important?

Would your wife allow you to treat your living room for better acoustics? ;-)

My system is in my living room and those tweaks would not pass WAF. So, room treatment would not be much of an option for me, except changing rugs. My living room is about 25x30 with wooden floor. 5x7 rugs in between the system and listening position.

Interconnects
Speaker cables
power cords

I've had the most success with a top down approach. Source interconnects first, preamp to power amp interconnects second, speaker cable third, and power cords last. This is not a hard set rule by any means though. A number of variables can potentially change this order of priorities. For example, if your power is very poor, then power cords may move higher on the list, etc. If you can, get a dedicated line for your stereo. That's a fantastic improvement that's not too costly.

IMHO, the order would be:

1. Interconnect from preamp to power amp.
(My reasoning is that everything goes through this cable.)

2. Speaker cables.
(Everything goes through the speaker cable too, after it goes through the above cable.)

3. Interconnects from sources to preamps.
(Your source signal should be as clean as possible. Note: if you only run one source, you might want to make this your very best cable.)

4. Power cables.
(This is variable, depending on whether you have dedicated circuits or not. If you do, then having better power cables is not nearly as necessary, IMHO. If not, than power cables, and possibly power conditioning, should be a priority. I have dedicated circuits, so I have not bothered with high end power cables. Fyi: I tried some, and it did not seem to make much of a difference.)

My two cents worth.

so the signal coming to the pre,#1your pre takes, what its been fed,prepares it,and off it goes to be amplified.the cable from the pre#2.The signal from your source and its quality or lack of, Will soon reveal itself to you .speaker cable#3 i use a pass/pre.I found in the 4 pre,s I tried,the sound was muddled,there was something missing in the sound,or adding to its performance.#4 power cables.the info used in the design of the high end cables, ,Its not a mystery.easy enough to DIY.Buy quality parts.Its all important .given,your source is where it begins.Where it ends,in your speakers,should be the best you can afford,IMHO.i now know there is some great audio gear,and it doesnt cost a fortune. Gotta know what to look for.Good Luck