Oh Jeeez, the single best is too tough dude. I can only itemize and comment on my favorite parts that have been drug around knowing that I really liked them. If I had to get it down to just one purchase in my entire tech life, it's kind of embarrassing: 60G OCZ Apex. It was my first SSD, was budget yet still pretty expensive; yet opened my eyes to what they can offer to a build and my love affair to follow to this day. If not for the WHS or needing capacity on the Main Rig (Raptor 1T) to work off of, I'd never have a need for a spindle drive in any build after that.
CPU: E8400. It's the Chevy 350 of CPU's IMO and haven't found anything that fit's it's niche for "do it all" until the i5 2500K, which I still don't hold above the E8400 over all in this timeline. The only reason I haven't ran here it is that s775 is officially dead. It's still doing duty in a buddy's charity build and will see it again this weekend for his checkup (and stealing parts I want). Solid Effing CPU and I've beat up on a few. 2500K 2nd, 925 Presler 3rd.
Sound: I'm with ya on the Klipsch Ultra 5.1's. I still rock them and will not replace anytime soon...probably die with 'em. I had a little heartbreak when it failed quickly, but with about $20 in tweaks with specific IC's that were at fault that thing rocks even harder and has been rock solid. I wear headphones a lot because I absolutely know I piss off the neighbors. These things have been the fly on my wall too. They've rocked through my bachelor days, pounded company parties, Swood a GF that became my wife 8 years later, and now have a boy that's almost 1yo that still gets to hear their beauty and soft tones. Tough call to not say it's my best purchase ever, but it's audio and not really a computer part (yes it's "related", I understand, and still think the SSD is a higher love affair)
HDD: 60G Quantum Fireball. This sucker was fast in it's day and am STILL running it. I think I bought it in 1999 (in a Mac store in Arkansas none the less) and it hasn't sat around not being used, ever. To still be kicking shows that this is the epitome of durability and how HDD's should carry themselves. It was part of my first real PC build and the only one still hanging around doing something. Not much, but something.
PSU: OCZ Powerstream 520. It wasn't just pretty in it's nickle case, but stood up to all my abuse I could give it; and this is before they even started to talk to PCP&C by a decade. It never died in the line of duty but to my own actions in 2009. I use to dissected it like an alien in Area 51 to see why it worked as well as it did, found it's Seasonic, and I do believe the world still revolves around Seasonic suggestions based on these minor findings. I finally killed it in 2009 and if I weren't so hamfisted w/ it, it'd be running in this garage computer today. Yup, curiosity killed the Angus Bull; yet it was done for the sake of Aperture science.
Memory: Man I can't even find it anymore, even on their website, but it's OCZ SpecOps Urban Camo SE DDR2 with the redneck chicken fence. It wasn't the regular 800 stuff but 866 or something odd that went far beyond that when I was a hardcore OC fanatic. I pushed it hard and stuff died around it far before it even shrugged. I have to check again, but I think I just shipped it out a week ago in build for a forum member. I'd feel more bad about it if DDR2 was still relevant. Either way, I've had 1866 (?) stuff from the same vendor freak out where these things just carried on in such a low rated bin. Those Reapers are clearly discolored showing I hit them hard and know I just ran the Urban Elites I've pushed harder in the last month. Hard core. Too bad they got out of the business really, but Mushkin slides in just fine.
Motherboard: No solid opinion. I have 3 great boards that still function and have their own stories to tell. Decades apart but are all in my solid category. I've never killed a board (bad boards kill themselves) and updates to a series are relative. If my hand is forced; the Abit BE-6 440BX/Slot-1 is the champ, Asus 925x 2nd (enter the new PCIe standard), Asus x58 3rd. Still all great in their own era so all equals IMO. It's all about the chipset really, then how the motherboard maker implemented it. Jumperless OC'ing AND it takes it? Abit BE-6.
I have other favorites but it's nickle and dimeing enough already. I can't put any of my favorite parts together in one build, and that's the sad thing about letting old hardware go to pasture. I will NEVER eBay/Craig's any of it and will use it in picture frame builds of yore unless it's a really good home.