When I think of great internet writing - not writing that happened to be put online but writing that is informed by and uses the nature of the internet ot its advantage - I think of qntm.
This book is the latest physical publishing of some of their short stories and internet fiction, collected and published in November of this year. As soon as I saw that a pre-order was available, I put up my money. I've purchased every physical print edition from qntm and have ready many more hours of their fiction online.
Aside: It's amazing seeing this as an example of modern physical publishing as well. They assembled this collection in August, uploaded the structure to Amazon in September, and it was printed and sent out in November. The book itself cost me only $6, and no inventory management is required - it's printed on demand based on incoming orders.
Some of the stories are from years ago (I Don't Know, Timmy, Being God is a Big Responsibility), and it's comforting to read them again. I think most of them hold up to qntm's more modern writing, though they aren't as confident in the reader.
The more modern examples, especially the collaborative fiction Google People, are so much fun. The seed of the idea sprouting between the author and contributors is exciting to watch and gives the sense of danger - watching something catch fire.
One of the reasons I really love qntm's writing is that they encourage the reader to look down the leaning tower of world-building assumptions and extrapolate backwards for themselves. In stories like Driver, the amount of backstory hinted at is energizing to piece together and filling in the gaps with your own ideas makes me feel a bit like I'm helping to write the story itself. It's as hard or soft science fiction as you like it.