Russian would be my choice, but it depends on what you want, Russian can be very hard for native English speakers, Russian has 6 case systems which will decline (change) the pronouns (lots of different pronouns) and noun endings based on where the word is in the sentence and its functionality. Ex. is it the subject, direct object, object of preposition. The 6 cases are the genitive, dative, accusative, prepositional, nominative and instrumental. The Russian language uses the Cyrillic alphabet which means the letters look different from what English speakers are used to (the Latin alphabet). Also one thing that makes Russian extremely confusing is the stress patterns, the stress can change from case to case and conjugation to conjugation, for example, some words are spelled the same but it you put the stress on the wrong letter, it can change the entire meaning of the word, for example, the word мука/мука are spelled the same, one means flour and the other means torture, the word is differentiated by which vowel is stressed. м(у)ка=torture and мук(а)=flour, see how bad that could turn out if one used the wrong stress? Another down putter, Russian stress is completely random, it has to be learned for EVERY word and and every form of that word. Russian word order is flexible because of the case systems, one could say in Russian, the dog bites the man, or the man bites the dog....as long as every word is in the correct case system, it will mean the same exact thing. This can make some English speakers confused since English has a set word order. (Subject+Verb+Other) Therefor, if you are not willing to immerse yourself in Russian culture or spend years studying, this language is not for you.
Esperanto is one of the easiest languages to learn....but it's useless but if you like it, go for it, it is very similar to Spanish and English.
As for German, it has the most usage, being the language of economics and one of the three languages of art, (Italian, French and German). Germany is commonly spoke, German, like Russian has case systems, though it only has 3 and they only decline with pronouns. It uses the Latin alphabet and the grammar is fairly simple in the beginning, in advanced German the grammar will become more complex but not near as much as Russian, as for stress....German has set stress patterns and is phonetic which means it sounds like it looks.
For me, French was always my favorite language, It's my native tongue and I love it, it is the language of art and the language of love, woman love it. The language has medium grammar, but again, nothing compared to Russian, though it can be harder than German, Pronunciation is easy, stress marks will denote stress and the end of every word will be silent, (with some exception, mainly when next word begins with a vowel). For example. Parler=to speak <
>, French, like English has many many exceptions.