These are the 3 foreign languages I speak, so I suppose I'm in as good a position to offer advice as anyone.
Spanish is the easiest of the 3 in just about all aspects. It's pretty simple grammatically and the orthography is very regular. Knowing French makes it even easier. The only problem I've found is that native speakers talk really quickly.
German may be superficially closer to English than Spanish, but in some ways it's very different. Yes, there are cognates, but are you really going to suss out that Verbindung means connection through the fact that it sounds like 'binding'? (compare withSpanish conección.)
People talk about German being a systematic and logical language but that's absolute rubbish. German does what it likes, and if you're not a native speaker you just have to wait till you get a feel for it. Hard and fast rules are very rare. For example, plurals can be formed in about 6 different ways, and most of the time you have no way of knowing which will be used. For example Der Apfel becomes die
Äpfel, das Buch becomes die Bücher. You have the exact same problem with gender in that it's not easy to tell what gender a word will be just by looking at it. Half of the grammar isn't even essential, it's just there - adjective endings in particular serve very little purpose.
Don't let me put you off it though - as frustrating as it may be linguistically, I've had plenty of chances to speak German in places I wouldn't have expected to, and there's a lot of good German philosophy out there if you get to a good enough level.
I may be biased about Japanese since it's my favourite language, but it really is wonderful. Grammatically speaking just about everything is there for a reason, and it is very flexible. The grammar is not difficult per se - no remembering huge conjugation tables - but very different from European languages. I personally liked this since it made learning grammar interesting instead of just muttering amo amas amat amamus etc. to yourself. The downside is that you will come across concepts which have no English equivalents and have to be learnt through practical experience, which isn't something that will happen when learning German or Spanish.
The difficulty of learning Japanese is massively overrated. As a spoken language I'd say it's pretty easy. The main problem is learning kanji, firstly because there are lots of them and secondly because they aren't implemented into the language as well as they are in Chinese - by this I mean that each kanji will have various readings depending on whether it's in a verb, on it's own or used to form a compound word with another kanji.
Learning kanji takes a long time unless you use the Heisig method which would have you learn the 'meanings' of all the 2000 or so 'general use' kanji proscribed by the Japanese government. I don't approve personally because most kanji can't be given a simple one word meaning that will actually be of any use, but many people learn this way.
The plus side is that Japanese is probably the most convenient language to learn on the internet with self study materials, many of which are free. Tae Kim's guide is probably the best thing to use for a beginner, and I haven't seen a comparable thing in other languages.
There is also a lot of Japanese language material on the internet in the form of anime, J-pop, manga etc. Unless you are really passionate about Tatort or Latin American soaps, German and Spanish can't offer that.
tl;dr Spanish is the easiest, Japanese is the most rewarding and 'convenient', German is tolerable if you have a reason to learn it.