From Easiest To Hardest, How Would You Rank the Difficulty Of The Romance Languages?
In my personal opinion being born into a monolingual English speaking household I'd have to say,
I've studied four of these languages, and I've dabbled in Catalan and Romanian, and I would say, for an English speaker, they would order like this:
Spanish, it's pronunciation is completely regular, except for 'mismo', as far as I can tell. The declension is very simple (o/a/os/as or e/e/es/es for the most part), and the verb conjugation is very regular (the only big exception are the root ablauts).
Portuguese, it's generally very similar to Spanish when written, though the plural is slightly more complicated (l-is and ção). The pronunciation is less regular than Spanish, though still quite simple.
Catalan*. I don't know much Catalan, but from what I can tell, the plural is generally regular (-/a/s/es). The conjugation also seems a little more complicated than Spanish and Portuguese, though I'm not sure.
French. It has the largest vocabulary shared with English, and the spellings of the cognates are many times identical, as the vast majority of Romance words came from French. The nouns are somewhat difficult, as there are a few strange plurals (al-aux, eau-eaux, etc.), and the gender of words are comparatively much more difficult to remember (o vs. a in most Romance languages). There are fewer verb conjugations in French, as the preterite and subjunctive past are only used in writing, and the future subjunctive is gone, though there are many more irregular verbs than in the other Romance languages. The pronunciation, though actually mostly regular, is quite difficult, due to the amount of silent letters.
Italian. Though the pronunciation, declensions, and conjugations are much more regular than French's, the cognates in Italian tend to look more different, as Italian split early during the evolution of the Romance languages. The plural is different from English (o/e-i, a-e), though as it does exist in English, this isn't too hard to get used to. The spelling is completely regular.
Romanian. I also don't know much about Romanian, but it is the only Romance languages to decline for cases, and the definiteness is conveyed by a suffix, not a article. It also has three genders instead of two (and I believe that neuter looks like masculine in singular and feminine in plural, so recognizing neuter nouns is non-trivial), so its nouns are definitely the hardest of the six languages. It has some (I'm not sure how much) loans from Slavic languages, as well as Hungarian and Turkish, so it probably has the most dissimilar vocabulary from English. From a glance, the verb conjugation system also seems more complicated than the other five.
Note that I learnt these languages in the following order: French, Spanish, Italian, Portuguese, and this could have changed the order (most likely, I think, the placement of French).
EDIT: Spanish and Portuguese also regularized the composite past, using 'haber' and 'ter', respectively, and don't use 'ser'. So that also makes them comparatively easier for English speakers.
EDIT: Romanian does the above as well.
About Romanian: the verbal conjugation (in the PRESENT tense) is indeed more complicated than in other Romance languages, but the other tenses aren't a pain in the ass, really. And speaking about tenses, Romanian has way fewer than its sisters (and it also only uses ONE auxiliary verb to build the composite past tense). So don't get scared of it. (I speak all these Romance languages). :)
It's hard for me to really rank the Romance languages, but there are two bands or tiers that I can distinguish:
Tier 1 (Easier): Spanish, Catalan, and Portuguese. Tier 2 (Harder): Italian, French, and Romanian.
Tier 1: These languages are all very similar, and once you learn one you will learn the others very easily. They don't have much irregularities in comparison to the Tier 2 languages. The grammar is a bit more straightforward and the vocabulary has a slightly more English influence.
Tier 2: These languages are slightly different. Knowing one might give you a bit of an advantage learning another one, but they do have noticeable differences. Their grammars have odd quirks and the vocabulary has a bit less English influence than Tier 1.