Your question at the end of your original post and your title differ a tiny bit. Are you asking about American English accent transfer to foreign languages or the prestige of American English itself?
The reason you think English doesn't transfer well to other languages is because its phonology and intonation patterns are quite distinct compared to the other popular languages of the earth. Check out my thread that details some
English speaker tendencies for some of these.
English speakers tend to freaking put stupid arbitrary stress wherever they want to and screw up vowels with a load of different sounds (that's because of our non-phonemic spelling and a high vowel inventory). Americans also tend to be more nasally, and not the pretty French/Portuguese deliberate kind either. Many times when I've asked foreigners how they think American English sounds, it involves exaggerating the nasal stuff.
German is pretty precise with its pronunciation and I feel that it is similar enough to many Slavic languages and even some Romance Languages (and Hungarian) that it probably isn't as pervasive (because it sounds like any typical foreigner). English, being a language that has deviated significantly from any other (Frisian, a cousin of Dutch, is said to be closest to English, but it's still pretty different even though they share some Germanic vocabulary in quite a precise way). Thus, I wouldn't say that it "takes the cake" for any classification.
German is thought by many to be "ugly", but I think this is just an assumption that one gets from hearsay. People with such an opinion have probably not heard what other languages such as Dutch and the varieties of Arabic sound like (not saying it's ugly, but Dutch does have many consonant sounds [anything close to the uvular fricative area] that many typically associate with "ugly").
Hope this offers some objective perspective, unlike many of the personal opinions here (not dissing on these, though).