A toss up between COSMOS or the Linux kernel, COSMOS because it's just amazing to see an open source kernel built on .NET from scratch (besides boot loader) and Linux kernel because well... Do I need to explain?

I used COSMOS 4 years ago in highschool and absolutely loved it, back then there was no real documentation and there were a bunch of bugs with the screen and user rings. Since then I've been watching it grow and it's become truly amazing, the ability to use C# with it really shines when making abstractions and trying to build a logical OS. 1000000% recommend it to anyone looking to get started with building OSes.

I'm not sure if it's still separated into a user and dev version but if it is I also recommend using the dev branch, it includes utilities and example OSes and abstractions and tons of useful features that make working with it a breeze!

@woodworks it's changed a lot but yeah they still have a user kit and Dev kit, it's constantly getting better too, the only things really missing are internet and USB support