For me i think its gota be the HDD's even though the PC itself is more valuable money-wise, its the HDD that makes the pc 'my pc'. Just plug my HDD's into another Pc and 'bam' my pc is back!!! how about you?
Hardware. All my stuff is backed up and all my music is ripped from my CD collection. Sure it'll be a waste of time to rip em again but they're still there.
Desktop - my HDD is probably more worthwhile than the rest of the hardware... this is only because the hardware is old and I'd have no issues with replacing everything. Macbook - this is a bit of a toss-up... I have all my homework on my HDD... but its a 1400 dollar machine... If someone was holding my macbook in one hand and the HDD in the other over a cliff... I really dunno which i'd choose. This actually really makes me want to backup my data... I wish 64 gig flash drives weren't 150 dollars :(
For me it's definitely the hdd even though I do have backups. The thing is, I do not take backups daily, more like monthly. So I might still lose something. Furthermore you can always buy 2nd-hand hardware if money is an issue, but you cannot buy your personal stuff back. And it did happen to me once in my early years ... hdd died with everything on it ... it was like a part of me died, lol ... but it taught me a lesson ... BACKUPS!!!
PC for me , hdd has nothing's on my hdd, just windows and apps, imaged well (from the beginning) already and no problems if hdd fails...if music/pics are lost, then let it be, nothing can be done... my cpu itself back then was $800 but those can't be judged, $1500 for my pc with all the hardware in there is nothing and cannot be weighted like this as they will depreciate very quickly? but if i was asked about what is more important out of my gadgets right now, it will be my Canon 70-200mm IS II which cost $3K, i can loose the PC anytime for those who says hdd because family pics are there, let me ask if you would pay $3K+ to get a professional hdd recovery company to backup your data? i highly doubt -lol