This is a big question, and here are some possibilities I'd suggest.
Most people don't need great hardware. You'll be okay with some crappy AMD APU, or i3 processor from five years ago most of the time. The key though is:
To the first point, if you run a heavy OS, like Ubuntu 20.04 with GNOME and all that, you'll probably want more RAM than 8GB. Trust me, from experience! However, if you go for a lighter distribution, maybe Lubuntu or Manjaro xfce, then perhaps you can go for 8GB or even 4GB (although 4GB... you're kinda pushing it there).
To my second point, an SSD will make most bottleneck moments much faster, resulting in an improvement in experience regardless of OS (you could run Windows and still feel the difference probably!), and it doesn't have mechanical moving parts, so it won't be as noisy and annoying. Trust me, it's worth it. I'd take 256 GB of SSD over 1 TB of HDD any day (just buy a 1 TB HDD later on if you really want it, they aren't expensive).
If you're planning on gaming, this is a different story, but this isn't really tips for gamers... but for someone wondering what is the "best value" for the "typical" user, that will run Linux.
Look for old Thinkpads. Of course, the dinosaurs are classics, but even if you get your hands on practically any Thinkpad, it'll be good enough to run most Linux distros, and it'll probably still be in decent shape.
If you have a bit more money, there are newer Lenovos that run AMD chips that can ship straight with Ubuntu (or Windows). Even if you don't want Ubuntu, if you don't want to pay the cost for a Windows license (as you don't plan to use it, in this hypothetical scenario), then that's a good choice, and the AMD chip makes it even cheaper - I think around $600.
In this case, look for a phone that can hook up to a monitor, and to a keyboard, maybe mouse, who knows. The idea being, you can turn your phone into some bastard work station, and you just need a monitor, keyboard, mouse, etc.
Get a Raspberry Pi 4, 8 GB.
Or start building a computer if you want... although as of 2021, GPUs are being scalped for ridiculous prices, so this isn't really the best time for PC building.
I don't really have anything against modern laptops, except that they are expensive, and for a Linux system, they aren't necessary to be so expensive. This isn't to say that modern laptops are bad, I myself have an X1 Carbon 6th gen (I bought before I was really into hardware, had ever touched Linux, etc.), and it's quite nice. If you can afford to buy one and find something you like, who am I to say ``don't get it". A new macbook (or maybe that Intel ``macbook") might be cool, I hear they have very nice battery life - for a ``minimalist" software user such as myself, that sounds attractive - imagine how long that thing could last with my software use cases! So there's a lot to look forward in the new. My main point is not to get caught up in hype, and recognize when something is actually novel, or when something is just marketing.
The real investment is to get one or two monitors... if you use your computer a lot for productivity stuff. You'll get far more done with a 2014 i5 chip and two monitors than a 2021 i9 chip and just your laptop screen. It's also, from the point of view of laptops/phones-you-can-plug-into-monitor/PCs a very modular upgrade - monitors are compatible with anything that will plug into them (although make sure your device you hook up to a monitor can support X number of monitors. My laptop can't support more than three screens, for example (I think)).
Another investment is a good keyboard. I'd suggest even a Lenovo 'chiclet' Thinkpad keyboard (wireless is cool), because I love the trackpoint. I also use a cheap Kumara mechanical keyboard with blue switches, which I like, other than I have to use an actual mouse with it. Also a good chair.