I voted for English. Pronunciation and spelling are massive irreglar, (or fiendishly regular, if you want to look at it that way). In fact, I still can't really, (and probably will never be able to) spell English.
Grammar's ok. The way I look at it, vocabulary is a nightmare to get right, although most non-native learners would never realise it. Because so many different words from different languages have come into English, they have all simly become specific, (small example - apple and fruit originally meant the same thing. Once fruit entered the language, apple changed to mean, specifically, an apple). In other langauges, (the best example I can think of is ancient Greek, but only because I'm a classicist), lots of words are vauge or conceptual. For instance, phrenas in Homeric Greek can mean: Chest, heart, mind, state of mind, emotion, wits, opinion. The precise words you use have a strong influence on your tone in English.
Also, we speak extremely fast and swallow a lot of sounds completely. And that's in proper English - modern langague or slang are even worse. Although inflection doesn't change the litteral meaning of words, like it does in Chinese languages, it essentially decides your entire attitude and meaning. People underestimate just how important inflection is in English.
I'm told that Japanese it quite easy, and is apparently very regular. Latin is also very regular, but the grammar can be an absolute nightmare. Ancient Greek is just horrible, (but quite fun ^_^). I never really enjoyed French, but I now suspect that was only because I was forced to do it and take exams in it. Now I'd quite like to go back and learn more.
Edit:
Well, we don't have genders for words. That's one thing in English's favour. And Japanese has lots of rules on what words acompany what attitude. For instance, a lot of men learn Japanese from their girlfriends and end up speaking a very effeminate, submissive form of the language, I'm told XD