It depends what you are trying to achieve. Is the aim true mastery and the attainment of a native-like pronunciation so that you might be mistaken in speech and in writing for a native speaker or is it about being able to read a newspaper and manage in a work environment? The former is difficult in every language and you have to be a bit of a fanatic to get there.
If we are trying to communicate in everyday situations with an intelligible pronunciation and read a newspaper, no west European language is really that difficult. It would take about a year of steady application and exposure (internet for instance). If you want to read specialist literature in your field but don't really need to speak the language, that is even easier.
To reach the level where you can comfortably and pleasurably read serious works of fiction is more challenging because you need both a wider vocabulary, including some obselete vocabulary and you need the meta language, all the background cultural information which the author is drawing on. Very up-to-date novels which include a lot of slang can be difficult to read too IMO.
Out of the western European languages, I imagine Portugese being the most difficult because I personally find it very difficult to hear anything out of a flow of Portugese speech (unlike French which most of us have learnt to some extent, Dutch, German, Italian, Spanish). Portugese is the one that my ear finds the most slippery - or elusive maybe.