A mattress set.
I read through this whole thread, and I have to agree on this one. We spend a third of our lives here, so to spend a little more to get a good one (or even top of the line one) is really worth it. I don't feel about anything else this strongly.
As for cars, I have a good one but I could have bought a faster one with more bells and whistles but for me, it's point A to point B. What I drive is ridiculously safe, perhaps the safest in class, yet not that powerful or stylish, but I am OK with that.
As for computers, I love my Mac minis and before that my iBook and somewhat used Power Mac did the job for what I needed. I didn't need to spring for a loaded 17" inch MBP with extra large screen to plug into on desk.
As for medicine, I find generics do a fine job.
As for wine, I have bought mid-priced bottles as good as the few high end ones I have had (saved up for) and to me, going all the way with great wine is not worth it as there are better things to spend it on. I also used to buy the exotic beers but found that I was OK with more common, less expensive beer.
As for musical instruments, sub-$1,000 dollar electric guitars, for instance, are only slightly less of a joy to play than the very expensive few guitars I have had. My off the shelf Fender American Strat I had in the 1990s was almost as good as the Fender Custom Shop strat in almost every aspect. Though I never flew, the Custom Shop strat had a flight case but that was more for show for most who bought that model. And to to sound better, I got more from practicing the instrument than a guitar with the latest and greatest innovations and electronics I can dial in on a decent amplifier or effects anyway.
As for tires, I get safe tires that hold up vs. ones that cost a lot that only show their worth in high speed applications. I don't race, or even go fast, so really cool racing tires are not for me.
As for clothes, something simple and durable does the job and as long as they are color coordinated, then fine. I have no great need for Versace suits or one of a kind hand made shoes to order.
As for education, I could have purchased an all-private education and both busted my butt to work harder and borrow more money, but I found in most cases, state funded education was just as good. Heck, it's what you make of it. I do see those private prep school and college alums talk about it ad nauseum for the rest of their lives, but I think there's a better way to spend $200,000 dollars, as in a house.