I took German in high school, so as a matter of familiarity, that would have to be one of my favorite foreign languages. Aside from that, I really enjoy listening to Japanese, though I don't actually know how to speak, read, or write it. I also like listening to and learning how to speak with different accents in English.

To answer your second question, I'll just copy my response to hanszyme:

Let me be clear that I don't yet have enough expertise to give you detailed advice on language learning in particular. That said, there are strategies for learning in general that apply to learning a new language. One big tip I heard I can't tell you how many times is to vary where and when you study day to day. It turns out that people tend to do better on tests when the testing environment is similar to the learning environment. So, by varying your learning environment, you can keep what you learn from being so dependent on the context of where you learn it.

Another thing you can do is try to speak, read, or write in your target language a little bit every day. Repetition is honestly one of the best ways for you to learn anything.