I like how everybody has come to more or less the same conclusions. I doubt that anyone will disagree with any of these answers.
There is nothing to add past this point, other than asking for specific models/brands. But let me try and diversify it a bit.
I don't think that the RX 580 belongs on the list, as it is not a recent card. Its age is the main factor that dictates its low cost. When it first came out, it cost significantly more - so it would be somewhat unfair to forward its current price. What's more is that it is not likely to stay relevant for too long.
For this reason I would say that the GTX 1660 is the better candidate for "best price/performance" as it costs only ~$30 more (where I live) and it is relatively new.
Concerning mid-range, most likely the RX 5700. Tech Deals recommended the 2060 as the best price-to-performance mid-range in his video, but I think that was only because the video did not feature the 5700. NVidia doesn't have a "middle" mid-range player at this point. The Super lineup has made both the 2060 and the 2070 more or less obsolete at that price. The 5700 has virtually no competition.
High mid-range would be the RX 5700 XT, the RTX 2070, or the RTX 2060 Super. These trade blows a bit better and they are all good for a high mid-range system. The best deal would be the cheapest of the three. However, $400 GPUs are not the first thing one thinks of when speaking of mid-range, so I'm not sure if they're relevant.
High end is no doubt the RTX 2080 Ti. Not that I'd be able to comment too much on the high-end market though, I left my $1000 GPU budget in my other jacket.
@BTGbullseye nice profile picture.