Michel Thomas' books are good if they suit your learning style, but Rosetta Stone is never worth the money. From my experience, the best books in general are Teach Yourself and Colloquial, although each language has it's own books. I have tons of books for many languages, so I could help you find materials if you needed it.
For one of your languages, you should definitely learn Basque.
But, seriously, only you can decide which languages you want to learn. I chose to learn begin learning Basque to fluency, not because it's widespread or has a lot of speakers (since it is neither), but because I love the culture and history, and deemed it worth it to learn. Everyone has their own reasons for learning languages.
You can't go wrong with German and Russian though, and Slovenian is pretty cool. He's an awesome song in Slovenian:
[youtube]http://www.youtube.com/watch?v=VDKzcm8WrxQ[/youtube]