Interestingly, the best GNU/Linux tablet is actually made by Microsoft. The Surface Go (first generation) is a tablet released in 2018. The performance is nothing amazing, but unlike most tablets, you can actually make use of the performance it does have; it doesn't feel slow for normal tablet tasks either. Besides that, it has a 10" screen, front and back cameras, a kickstand, stylus support and a detachable keyboard — but the latter two are sold separately.
There are five models, and I've got the middle one:
4 GB RAM, 64 GB eMMC, no mobile data
4 GB RAM, 128 GB NVMe, no mobile data
8 GB RAM, 128 GB NVMe, no mobile data
8 GB RAM, 128 GB NVMe, LTE
8 GB RAM, 256 GB NVMe, LTE
All of them use the Intel Pentium Gold 4415Y CPU; you can read the details yourself, but we're not focusing on raw performance here. An upside to this weak CPU is that the Surface Go is fanless and completely silent.
Design
The tablet itself weighs 0.52 kg and it's 8.3 mm thick. The whole body is magnesium; it feels like a quality product. The screen bezels are a bit thick, but that's not a problem for me and it also allows the screen to be fully rectangular. The kickstand is very useful and stable.
A minor issue is that it only has one USB-C port, but there's also a proprietary magnetic charger port, a headphone jack and a microSD card slot, which is hidden under the kickstand. You can also charge over USB using any regular phone charger.
The keyboard cover is made from some sort of fabric which is interesting, and mine had broken edges, but I don't mind. It's good, I'm not very used to it yet, and it's smaller than a laptop one. It's still very useful and it also has the arrows and function keys. The trackpad is what you would expect, but I prefer to use the touchscreen or stylus as it's faster. A mouse would be the fastest, but I'm not going to carry one around. In case you don't want to use it, but still want to keep it attached, folding it behind the tablet automatically disables it.
The stylus is good. I'm not very experienced with styli, but it's thick, which I like, has 4096 pressure levels, a button on the side and another one on the top, which is also an eraser. It runs on a AAAA battery.
Compatibility
I used Linux Mint 22 MATE, and after installing the linux-surface kernel using the provided instructions, everything worked. Krita recognised the stylus, including the eraser end, out of the box.
Scrolling with the finger on the touchscreen works very well in GTK apps and in Chromium, and you can hold to select. The stylus always selects text like the mouse. However, scrolling by swiping isn't possible in Firefox, Electron or Qt apps, where you must use the trackpad, scrollbar or arrow keys.
Otherwise, it's what you'd expect from a regular GNU/Linux system. The battery lasts about 4-6 hours of normal use which is more than I expected.
Recommended software
Krita: for painting
Chromium: web browser, better touchscreen support
Touchégg: touchscreen and trackpad gestures
Onboard: a fine on-screen keyboard
Everything else Linux Mint comes with
I have yet to find a good stylus notes app.
Conclusion
If you want a portable machine with GNU/Linux, this is probably one of the best options for now. The advantage to being made by Microsoft is that it's a polished, quality product, which is more than a developer's toy.