Although the internet is outrageously huge, there seems to be no definitive answer, for it seems there are many factors. Thinking about it, it surely depends to some extent on a person's native language.
For example I, as an English speaker, would most likely find Dutch easier to get to grips with than a Chinese person trying Dutch as his or her first foreign tongue. For a start, Dutch uses the same alphabet as English so that means I don't have to worry about writing systems. Also, many Dutch words are spelled exactly the same as English ones, even if pronunciations can be somewhat different.
I personally believe it is possible to learn ANY language comprehensively. The only variable is time. Some languages may require more time and effort to become conversational than others, but one can still reach that point all the same.
Still, the question itself is still one that holds an interest with me, so here are some common languages that keep cropping up in response to this query from native English speakers:
• Arabic - Although one of the most widely spoken languages in the world, it's vocabulary bears virtually no similarity to most European ones. It's writing system is also very foreign and is read from right to left too. Also, there are wildly different dialects, so someone speaking Arabic in Morocco may have great trouble understanding someone from Saudi Arabia.
• Mandarin Chinese – Another that always pops up. Again, it’s writing system would flummox any newbie, plus it has four tones to compound matters. However, a billion Chinese people can speak it perfectly well!
• Basque – Although the Basque region comprises part of France and Spain, it’s tongue resembles neither French nor Spanish. It has 24 cases and makes wide use of prefixes and suffixes to drastically alter the meaning of words and sentences.
• Icelandic – Often described as one of the hardest European languages to learn, Iceland’s official tongue has rather complex grammar and again bears little resemblance to English. However, this amazing man was able to learn enough of it to be interviewed on Icelandic TV in just one week!
There are many others that often get mentioned too, such as Japanese, Hungarian, Finnish and Navajo, but again I believe that all of these languages can be learned to fluency if one can dedicate enough time into doing it.
This great diagram I stumbled across gives a very rough estimate as to how long it would generally take someone to become proficient in a number of different languages (The image is a lot smaller when attached, so click the link if you can't see it properly):
