Since I helped hijack the thread, I try to return to the original point, sort of...
1) The cable that makes the biggest impact is... the last one you changed. Before you completely dismiss this dopey statement, here is my point: your system as a whole is only as good as its weakest point. Assuming you have a system that is not currently in relative balance in terms of the absolute capability or resolution of each piece, replacing the weakest link should provide you with the greatest benefit. I think this is how most of us operate, moving piece by piece through upgrade cycles. So if you have capable electronics, great speaker wires and interconnects, but are using an unshielded sixteen gauge power cord. changing the PC should make a huge difference. If the reverse is true, and your speaker wire is not supporting the signal it is receiving, replacing it may provide the greatest PERCEIVED change in performance. This doesn't mean that the speaker wire is the most IMPORTANT piece in the chain, just the one that was holding your system back in this instance.
2) I have spit a lot of martini in vigorous discussions with audio enthusiast friends about whether the world is source centric or speaker centric (and I find that the speak-o-philes in my circles tend to laugh at my affection for vinyl - so what does that tell ya?). The source centered argument was leveled earlier in this thread at cables, start building your system from the wall out, with speaker wire and speakers being the last pieces in the chain. I buy into this logic in theory, but in practice my investments in ICs and long runs of decent speaker wire far exceed my investment in power cords. I am coming to the party and will purchase a better PC for my system next.
So, "Which Cable Makes the Biggest Impact?" The next one you buy that works better in your system than the weakest link it replaced. In a generally level system, I would have to say the the most important parts are the ones closest to the AC outlet, because you can't add something that has been lost, or correct something later in the chain that has been messed up along the way (except perhaps, a good power conditioner, but occurs very early on in the chain).
PS - I have a modest system that is "creatively" and economically tweaked to milk the greatest performance out of it. I have noticed changes from nearly every major addition of electronics and cable. I found that modest PC, IC and isolation upgrades offered immediate and startling improvements in sound quality, while long runs of speaker wire took longer to show their value (literally months of break in). Most professional reviewers make apologies for using their $2K power cords for the review of $500 gear, but then go onto acknowledge that it makes a big difference in the performance of even entry level HiFi gear. From my experience, what I read and what I think (oh oh), cable upgrades can benefit all systems and should provide noticeable improvements in well set up budget to mid Fi systems, and that the better your system, the more the subtleties and inherent values of different cables will be expressed.
Finally, I have to agree with whoever said earlier in the thread that the most important component in your system is your room, but that is off topic...