As someone who spends a lot of time travelling, I'd rank Istanbul to have the best döner in the world. You find alright stuff in Germany, acceptable options in the rest of Europe, and I've never failed to be disappointed by döner in the US (which is a shame, because I keep trying it because it's a top 10 good for me)
This is frankly odd, given that for most cuisines I can point at some other place surpassing the original. The best Italian food I've had has been in Chicago, the best Mexican food I've had is in San Diego. You can find extremely good authentic Chinese food in the Bay Area if you know where to look, but I can't say I've covered enough of China to confidently decide a winner there.
Yeah I can't really tell why it's so challenging to find good levantine and Middle Eastern food in Europe (even in places with lots of Turkish and Arab immigrants). Turkish döner vs Arab shawarma is more of a matter of taste (the döner is fattier and the dairy based sauces make it even fattier which is not to my taste but is not objectively bad thing) but I don't get what's so hard it making it that you can't get good results outside its homelands.
At least in Germany I suspect it has something to do with Germans considering it cheap fast food and as such are very price sensitive - there's only so much you can do when you have to cut every expense as much as possible.
This is frankly odd, given that for most cuisines I can point at some other place surpassing the original. The best Italian food I've had has been in Chicago, the best Mexican food I've had is in San Diego. You can find extremely good authentic Chinese food in the Bay Area if you know where to look, but I can't say I've covered enough of China to confidently decide a winner there.