I'm seeing this old topic and I would like to give briefly my opinion, because it interests me a lot, basically due to the fact that I speak the great majority of romance languages and all the most important ones, and I also teach few of them, Spanish, Italian, Portuguese, Catalan...
It's true what has been said about Romanian, but it's also true on the other way: knowing other Romance languages, especially Italian, due to the many similarities, Romanian is for me not so difficult at all.
And it's also true that it depends on the point of view: it is different if it is a German or a Russian or an Italian, an Englishman or a Romanian who says that...
and it also depends on which languages you already know, etc...So, very difficult to make such a ranking, which would not be totally reliable for sure, but I like that...
Based on my personal experience and what a lot of people has said to me, without doing a deep analysis now, I would list them as follows, trying to see it not from my own point of view:
Perhaps ROMANIAN is still the most difficult, due to the cases, the articles (definite and indefinite), to the plural...
But maybe a Serbian would not say the same...That's why I don't take into account the Romanian words of Slavic origin: they have not the same difficult for everybody.
Second one maybe CATALAN: 4 groups of verbs (cantar, córrer, admetre, omplir), but really really a lot of irregular ones and also with other suffixes (-eix); complicated pronouns system: ES pentina; S'ha llevat tard; va llevar-SE; not so easy the plural too; particles: en, n': se'N va; different pronunciation of e and o depending on if they are stressed or unstressed (in this last case, they become respectively a and u)...
Then, more or less same difficulty level as Catalan, I would say SARDINIAN: verbs are pretty regular, even if not in the participles (3 main groups like in Spanish, Italian, Portuguese and French), plural too (the same as Spanish), but the syntax is often uncommon to the other romance languages (and not only to them), which influences the Italian spoken in Sardinia too (faendoddu fia = "facendolo stavo" = stavo facendolo, estaba haciéndolo; fae' puru d'aias pòtziu = "fare pure l'avresti potuto" = l'avresti pure potuto fare, también podrías haberlo hecho); a lot of particles, (ke, nde, bi...), sometimes even 2 in the same sentence: si KE ND'at pigau duas (se NE ha preso due (feminin), se N'ha agafat dues, in both Italian and Catalan just one particle, se ha tomado dos, se pegou duas/apanhou-se duas, in Spanish and Portuguese there are no particles.
In addition to that, pronunciation is very complicated too: according to the word that follows, p becomes something between b and p, c something between g and c, b something between b and v (like in Spanish), t something between d and t; also at the end of the word, you never know how the s of the plural is pronounced: for example, the article sos/sas could be "sor/sar (or sol/sal, according to the region)", "so'/sa'" or "sos/sas" indeed, and one vowel can be added at the end of each word.
One more easy example with two words common to many languages: su cane/sos canes (the dog/the dogs) is pronounced "su gane/sos cànese" or also "sos canes" indeed; su gatu/sos gatos (the cat/the cats) instead is "su 'atu/sor gattos" or "so'gàttoso".
And also, the apostrophs' system is very free, like or more than Catalan and much more than Italian.
There is a lot of unique vocabulary too, like the months for example: làmpadas (June), triulas (July), cabudanne (September), ledàmini (October), Sant'Andria (November); also chenàbara (Friday), Paschighedda (Christmas), petha/petza (meat), tziligherta (lizard), tzilibriccu (grasshopper), tzintzigorru (snail).
I would give one special mention to SASSARESE, which is a language between Sardinian and Corsican-Gallurese.
It has most of its features from Sardinian, but grammatically is more akin to Corsican, and as a result to Italian.
Its pronunciation is as difficult as the Sardinian one, he has also the cacuminal sound DD and the guttural GG, not really common. And o if not stressed becomes u, as well as e becomes i.
But more than all, it has a unique sound, that elsewhere is just to find in Northern Sardinian, transcripted as "sth" or "lth", which is impossible to describe, also phonetically, because it is pronounced like a kind of whistle...and it's also impossible to imitate.
Then I would put PORTUGUESE, which has a verbal system more complicated than Spanish, and maybe more than all the other romance languages too, with some unique features like future subjunctive (in Spanish it is disappeared now) and personal inifinitive...; the pronouns are also not so easy (even if more than in Catalan), particularly the contracted ones mo, to...but they are disappeared in Brazil; plural (with forms like répteis, utis, corações, pães, mãos) is more difficult than in Spanish, Sardinian or Italian, but not so impossible; in the pronunciation, e if not stressed is almost not pronounced (Portugal), or pronounced as i (Brazil; telefone being respectively "tlfon" or "telifoni"), and o, if not stressed and at the end of a word, is always u.
Following Portuguese, same level more or less for French and Italian.
FRENCH is so difficult for many, for me is not as much because, even if it is right that the pronunciation has its own rules and is not consistent with the writing at all, it has anyways not so much exceptions to its rules as English, for example; in addition to that, the fact that in many verbs the pronunciation is the same for the most persons, as well as in the singular/plural, helps a lot.
The verbal system has got less times than other languages, and the plural is also not complicated.
Writing French is much more difficult than speaking it.
ITALIAN, in comparison to Spanish, has more irregular verbs (with suffixes, like in Catalan, for example by preferisco), even if the endings are easier; then, it has 7 articles, the most between all the romance languages; plurals are more complicated then in Spanish or French, with endings i, e, or staying as in singular, plus exceptions: il dito/le dita, l'uovo/le uova, l'osso/le ossa and, different meaning, gli ossi, etc; like the other romances, it has stressed and unstressed pronouns and therefore two forms of dativ, a me und mi, like Spanish, Portuguese, Sardinian, Catalan...; it has particles: ci, ne...together they become "ce ne/ce n'".
The pronunciation, nevertheless, is to some extent easier than the French one.
Last but not least, SPANISH.
It may be the easiest one to speak it a little bit, but surely not so easy to speak it properly, due to his HUGE vocabulary, surely bigger than the Italian one, for example, and it has also a lot of peculiar words, for example of Arabic origin, like alfombra, alforja, albarán...
Anyways, its pronunciation, like the Italian, doesn't have vowels pronounced differently depending on if stressed or not, and not so many consonants with double sound like Sardinian for example, except b/v and ll/y.
The Andalucian and South American pronunciation anyways is more complicated, with the dropping of s in implosive position (los amigo' or los amigoh, loh perroh...etc). It has also the sound "th", like in Zaragoza, almost unique between the romance languages, and a very strong (in Spain) anspirated j/ge/gi.
The verbal system is very large, but with not so many irregular verbs as in other languages; the double dativ and double accusative (in South America) are surely a problem for speakers for exemple of German or English.
Anyways the pronouns are not so difficult, there no contractions (therefore no apostrophs, like elsewhere just in Portuguese between the romance languages) and no particles.
Greetings
