Spanish is much easier to pronounce, and probably slightly easier when learning grammar and vocabulary also.
Having said that, personally I find French easier, but as I'm half French I'm somewhat biased.
Italian is harder as there are neuter nouns (as opposed to just masculine and feminine). And Polish is just rock solid as you don't only have to conjugate verbs but also nouns and adjectives.
Source of reference :
I have studied French as far as 1st year of BA at uni. Plus, as I say, I'm half French.
I have an A level in Spanish
I have a GCSE in Italian
I have studied Polish in Polish school as a child, and also did a beginner's Polish course at uni