Ah, I see someone had mentioned Latin, which, next to ancient greek, is a beautiful language I wish to fully learn and comprehend, as if you know the two, etymological questions on many different tongues become clear and simple to understand.
I guess those two are the languages I feel attracted to the most. Of course, it is always a question of taste. English is simple yet has high potential, Russian is simple and has interesting roots. Japanese is more of a language I would learn if I were more of a utilitarian, as, ahem, not much japanese literature and art is translated perfectly in western languages.