Which phones have the purest Android?
tl:dr; Google phones have the purest Android. They come with Google apps
In Android ecosystem, phones launched by Google are stock Android (aka vanilla). It's the purest form of Android because Google developed the OS and tightly integrated with the hardware (earlier Nexus and now Pixel). They come with Google apps, you can't avoid them, unless you root your device and uninstall.
If you are looking for a phone that is running Android LineageOS -XDA launched a phone that ran LineageOS /Ubuntu out of box. Such devices are more of an exception to the rule (running LineageOS out of the box, as you mentioned in the last paragraph of the question, though LineageOS doesn't qualify as pure Android)