I've stuck to ThinkPads. They are great....or were great. They recently changed the keyboard (bad) and also got rid of the physical mouse buttons (NOOOOOO! BAD LENOVO!).<p>With those gone, there isn't anything particularly compelling about the current ThinkPads. In that case, just pick most anything and go with it. Ubuntu should run great on most anything these days.
Are you aware that you can install a Linux distribution on just about any hardware? System76 are OK, I've never been wowed by them. Realistically, I would just go with a Thinkpad, unbeatable keyboard and great build quality (though lately Lenovo has been messing up on both of those)
I just bought an x1 carbon and loaded it with ubuntu. Just about everything worked out of the box, less microphone mute button and sleep button. It's pretty flawless and hands down the fastest machine I've owned. Battery life is also awesome (5-6 hours).
ThinkPads have a historically good reputation because kernel developers tend to use them. But as trouserpants says, almost all of the machines from major manufacturers will work fine.
If you want some 100% no issues working, just get something with Intel CPu and GPU. Although the free Radeon drivers are pretty good now too. Avoid some kind of integrated GPU+extra discrete chip combination. Otherwise there isn't really anything special to worry about or to buy a special "built for Linux" machine (which I consider marketing bullshit).
Honestly, I just install it and don't pay much attention to the brand of the machine. The issues I have had installing an OS seem to be about even whether I am installing Windows or Ubuntu.<p>The only issue I had with my Toshiba Satellite P855-S5312 was getting around secure boot, which is easier now. I've been running Ubuntu on it for about 15 months with no issues.
I use a ThinkPad. Mine is a t530, and a good balance of the things I want. 16gb ram and a discrete video card are nice.<p>Best is vague. Best bang for your buck? Best ultra portable? Best workstation? Best battery life?
I have owned a couple of System76 machines and they have always been pretty good, I currently have the Darter and I have no complaints.<p>If you want something small and portable my Asus X202E works well.