I think many more countries are much worse than the UK, although I agree the American notes are dullsville and stuck in 1928 or whenever they were designed. Beware, I will again reach into my own personal preferences, because I actually have an opinion.
My least favourite are 2 types, 1990s ex Soviet/stan notes and Indonesia.
Between 1990 and 1993 when the big communist states like USSR broke up, all the new countries issued banknotes and with the exception of the Baltic states, all were cheap and truly awful notes. Some of the stan notes looked like they were printed on toilet paper and tissue. They all had cheap basic designs that were almost the same from the 50 centniks up to the 100,000 Rubleniks. None were stable and they added zeroes fast. Belarus is a good example, the 50 Kapek (Love all the creative spellings of Kopek and Ruble) looks similar to 100 Rublenik, Rubli, Rubalachim, Rubelskis, Rublekovs, Rub(Add stereotypical ruskie sounding suffixnik), or whatever they were called. Also in the firing line were the Yugoslavian notes that used the late Communist era designs and added more and more zeros.
Then you have the sheets of coupons which were basica to the extreme. Communism was such a dumb system and these countries had no solid metal backing for these worthless tissue paper currencies, due to communism banning gold and silver as it was too bourgeois and not communist enough (You had to be bartering carrots and wearing the same babushka outfit since 1961).
Of course the more stable countries like Estonia, Slovenia, Czechia etc got more well designed notes and more stable currencies after about 1993/94 but places like Tajikistan, Ukraine, Belarus, Serbia and all the more hardcore places remained hilariously awful. Even Russia had funny notes before the 1995 redesigns. They just issued more 1961 USSR notes with more zeros and a bigger Lenin portrait (Even now this communist seems to hold sway in “Democratic” Russia).
Indonesia as all their notes look cheap and horrible (Although the newest series looks a bit more sophisticated, the people on them are ugly and the highest note is worth around $5 US). As a kid the Indonesian notes were everywhere and nearly all worthless with the 100 and 500 Rupiah issued when both were worth a few cents. In the 1960s they issued ultra low value notes and a 1964 series of 1 to 50 sen is now the lowest face value of all time, showing a “coolie - very archaic and racist term for a peasant”. “a peasant girl and a soldier”. Even now these notes have zero value and despite some nice images of mountains and orangs on some notes, they are overall ugly.
I am also not a fan of German inflations (Too many, too plain, too much gothic lettering), Hispanic Inflation - all those notes of 10,000 neuvos pesos crusados etc with some guy with a moustache sombrero and a bandolier on them.
I love coins. Especially silver, gold and anything really old.
Member of the Royal Numismatic Society of New Zealand and the Auckland Numismatic Society