The most daunting European orthography to me remains Danish. Danish has 9 vowel letters and 26 vowel phonemes, and it doesn't use digraphs like English, Dutch or French. It does use silent E for long vowels, but not always. 8 vowel letters have 2 or 3 possible short pronunciations. What a nightmare. In comparison, American English has 6 vowel letters, many common digraphs, and only 16 vowel phonemes. French is even easier in this regard: the pronunciation of every vowel letter and every digraph is
completely deductible. You will never wonder: "How do I pronounce this vowel?" like you do in English. Unless maybe in some loanwords? But many languages have that problem.
Compare that with English: ea sounds different in the words ocean, heart, ear, wear, meat, bread, steak. And then there is oo, ought, ow etc. French spelling lacks this madness.
So I would rank them like this: Danish > English > French > the other European languages
I have a bit of a problem characterising the complications of French spelling as "subtle".
The complications of French spelling:
-You have to add silent s for plurals, but it follows the same rules as silent e which is often pronounced, so it is not really an extra problem for learners
-Verb endings: the tu conjugation always gets an extra s and the ils/elles conjugations always get an extra nt. The je conjugation always gets an extra s except for -er verbs. The endings -er, -ez and -ai all sound like é.
-Many double consonants are useless
-C/Ç or S/SS? G or J? English has the same complications.
-tion or -ssion? English has the same complications.
-An or en? English has the same madness (
part 1,
part 2)
-In or ein or ain?
-È or ai or e? Ê or aî?
-Ô or (e)au?
-Eu or œu?
-Liaison: -t or -d? -s, -z or -x?
This is pretty much it. There might be extra complications depending on your accent, but those can't be helped.
You cannot make a similar post for English. Wikipedia has tried though.