Most people I know haven't even heard of it, but mcmaster.com is the best e-commerce site I've ever used.
McMaster-Carr is an industrial supply company. They sell nuts, bolts, bushings, bearings – pretty much anything an engineer needs to build stuff. I've purchased from them dozens of times over the past few years, both for personal and school projects.
But what makes their website so great? And why should an industrial supply company have the best e-commerce site on the internet?
mcmaster.com is great because it does what it needs to, and nothing else.
First, let's look at the visual design of the site. Minimal, mostly grayscale, with accents of green and yellow. There are no popups, animations, banners, carousels, or videos – just a calm, static page with categories and a search bar. Even the images are grayscale, to avoid inadvertently catching your eye.
It's not the most visually stunning site, but that doesn't matter here - McMaster has chosen function over form.
A user's goal when they visit McMaster-Carr is to find their part as quickly as possible. The website is designed entirely around that fact. Users rarely come just to browse, so there are no AI recommendation algorithms, featured products, new arrivals – that doesn't make sense in this context. People visit McMaster-Carr with high intent to buy a specific part, that's it.
So how do we get from the 700,000 products in their catalog down to one part? Here's what I do.
Let's say I'm searching for a bolt:
I type "bolt" into the search bar
McMaster shows me several subcategories: hex head, socket head, set screws, etc. I'm looking for socket head, so I select that one.
Now I move my attention to the left nav bar, which shows me several filtering options. Bolts are commonly specified by their thread size (e.g. 1/4"-20), and their length. I'm looking for a 1/4"-20 x 1" bolt, meaning that the bolt's diameter is 1/4" and its length is 1", so I select these filters.
There are over a dozen other filters, such as material, hardness, and head size. Once I've applied enough filters, The main search window shows individual items, rather than subcategories. Here I can select an item and add it to cart.
McMaster's search interface is the main reason for its design superiority. Everything on this page is designed to get you to your part as quickly as possible. The filter sections are simple and elegant, providing schematic illustrations when necessary. The illustrations are always simplified to convey only relevant information, as to not distract you from the focus of your search.
Results pages also show helpful drop-downs which explain the parts you're looking at. It's like an engineer's handbook and catalog in one. Engineers are often looking up terminology on the fly anyways, so having this information embedded into the site saves valuable time.
McMaster's filters are not only useful for a targeted search, but also for deciding what it is you want. Sometimes I'll search with only a general idea of the part I need, and then I'll use the subcategory descriptions to determine specifics. For example, I may know that I need some type of lock washer, but I'm unsure which one is best for my application. I can use the images and descriptions to decide on the right configuration.
McMaster is able to provide such intuitive searching and filtering because everything that they sell is highly legible – it's all defined by quantitative specs. There is nothing intangible to deal with, like brands, product photos, or other marketing fluff. Even still, they do a much better job than other industrial websites like Grainger, DigiKey, or Home Depot.
As a point of comparison, Amazon does a terrible job of filtering items. Amazon has an order of magnitude more products on its site, which admittedly makes the job a lot more difficult. However, even generic filters, like price, suck. I won't get too far into my disdain for amazon's UI design, other people have already written too much about it  and that's not the point, but it's interesting to contrast McMaster with what everyone sees as "the" e-commerce site.
Take Amazon's price picker: Why is it two text boxes? Why not a slider? This has always annoyed me, since it's much easier for me to drag a slider than to manually type in my max price. And the quick-select ranges are literally the exact same no matter the product. If I search for a pen, nearly every result I want should be under $25. If I search for a trampoline, every result I want should probably be over $200. What the fuck?! I guess this somehow won the A/B test, but I can't think of a reason why.
Finally, one of the most brilliant parts of McMaster's product is that for nearly every part, they have a CAD file that you can instantly download into your 3D models. Mechanical engineers mock up designs in CAD programs before actually building them, and having access to pre-modeled parts saves time. (Imagine having to manually model all your nuts and bolts.) McMaster even has extensions for popular CAD programs which allow you to import part files directly, instead of using their website. This makes engineer's lives 10x easier (not to mention making them more likely to purchase from McMaster-Carr). The closest analogy to this is AR try-on, but that's not even very accurate. The point of AR try-on is to determine whether you like the item you're about to buy, whereas the point of McMaster's CAD downloads is to speed up an engineer's workflow. In most cases, they already know which part they need, it's just a matter of completing the CAD model before they can start building the real thing.
Mcmaster.com is nearly perfect. It's a website that would make Steve Krug smile. My only suggestion would be to make the search bar on the home page more prominent. It's visually overwhelming to comb through dozens of product photos, so I pretty much always use the search bar to start narrowing down items. The main area of the homepage is effectively dead space, while the search bar is relatively tiny. New users might miss it, wasting time.
I decided to write about McMaster-Carr because it is so rare to see such careful thought go into an industrial web app, far removed from the commotion of silicon valley, web3, D2C, and the other typical subjects of pixel perfection.
Mcmaster.com is a product that understands its customer. The minimal, functional design allows users to find their parts as quickly as possible, nothing more or less. It's an unexpected reminder to not get lost in the allure of smooth gradients, 3D animations, or slick fonts, and instead relentlessly focus on what it is your customers really want.
"If something is 100 percent functional, it is always beautiful...there is no such thing as an ugly nail or an ugly hammer but there's lots of ugly cars, because not everything in a car is functional...sometimes it's very beautiful, if the person who designed it has very good taste, but sometimes it's ugly." 
 "Jeff Bezos is an infamous micro-manager. He micro-manages every single pixel of Amazon's retail site. He hired Larry Tesler, Apple's Chief Scientist and probably the very most famous and respected human-computer interaction expert in the entire world, and then ignored every goddamn thing Larry said for three years until Larry finally -- wisely -- left the company. Larry would do these big usability studies and demonstrate beyond any shred of doubt that nobody can understand that frigging website, but Bezos just couldn't let go of those pixels, all those millions of semantics-packed pixels on the landing page. They were like millions of his own precious children. So they're all still there, and Larry is not." https://gist.github.com/chitchcock/1281611