Which cable do you think is most important?

The one that connects to the Forums/Cables.

Ghstudio is right. Get your room as perfect as it can be and then worry about cabling. The speaker/room interaction is the most under-tweaked aspect of 99% of the systems out there. If you do not feel capable of doing this, consider having a professional firm come in and help you. Rives is well-known on this site....

-RW-

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.