I class the XR as the mainstream iPhone aimed at the mass market and the XS is more niche due to its premium. The XR is far from budget despite being cheaper than the XS line and is more the standard I would say or mid tier.We can pick a different adjective if 'budget' doesn't sit right, but out of the three models launched in 2018, the XR is definitely the least expensive and most feature starved phone of the three. My point is that when someone is making a claim that a year old phone (the 8 Plus) is the best iPhone ever, there's not a lot of validity in comparing it to the least capable 2018 phone to make a point.
The discussion can arguably change if someone was talking 'value' (which is entirely subjective, of course), but the discussion wasn't around value, it was around quality.
I am uncomfortable claiming any particular model is the ‘best ever’ as I know it’s purely subjective. Lots of people I know would say the iPhone 4 is the ‘best ever’ due to what it offered at the time, which had far more of an impact than any of the recent offerings. The 8+ is the best iPhone I’ve personally owned but I’d never expect everyone to share the same view. This year for me it was the best iPhone for my needs due to its spec, and price. It keeps up with 2018 models even if the fashionistas don’t think it looks ‘as cool’.