Is there a sound in the German language that is similarly difficult for English speakers as th is for German speakers?
The ulvular trill of the German r is tricky for us. I can do it after a vowel but not very well at the start of a word.
The ch in ich takes a while to master, but mostly because beginners tend to assimilate it with English sh, not because it's terribly hard. The ch in nach is easier.
ü also takes a while to get to grips with.
I consider none of the other vowels (and diphthongs) difficult to articulate for an English speaker, but for a beginner it's natural to substitute them with a similar but different sound that's more familiar. In the same way, a English speaker with a non-rhotic accent may have a tendency to pronounce word-final -er as a schwa as in those varieties of English.
As a Brazilian, my native language is Brazilian Portuguese. I found it much, much easier to learn the German phonemes than the English ones, both to understand and to pronunce them.
However, there are some phonemes that are a bit tricky in German. The umlauted vowels are a bit of a challenge. However the
ch is way harder. I can say I did not "get it" yet.
r (uvular trill) is somewhat hard, too, but not impossible.