I'm a language geek - I've studied Italian, Portuguese, Greek, Hindi, Urdu, German and Greek - in addition to having spoken English and Spanish naively.
Spanish has the huge advantage of being very useful in the United States.
German is a Germanic language, like English. It does not make the sciences any easier.
Italian is great because it is very similar to Spanish, but with much simpler grammar structure, so somebody who might have difficulty in learning languages might find Italian easier to grasp. It is also the closest living language to Latin, and a very strong understanding of Italian can be helpful for complicated English-language vocabulary.
Portuguese is like Spanish, but can be very, very difficult to pronounce. Though mastering Portuguese is a great way to master various vowel sounds, something that can become useful in other languages.
Greek can build vocabulary skills in English, given the strong number of Greek-rooted words, but is complicated in its grammar.
Hindi is a great language to learn because it is very simple and straight forward. It uses a different alphabet, but is otherwise Indo European, so it is not difficult to grasp. An English speaker can become proficient in Hindi with one year's study; Arabic or Chinese needs easily 2-4 years.
The "usefulness" of European languages is ever decreasing as English-language education has become increasingly mandatory in Europe. The average Scandinavian, IMO, speaks better English than most Americans.