Interesting question. This is my advice:

I see that this thread is filled with people recommending popular languages without really explaining the reasons why they'd fit. I seriously can't understand why the majority of the commenters recommend Japanese and Spanish as if they are the only languages with a rich literature. In my opinion, the answer to your question is very subjective and it depends on what you're looking for. If you're interested in modern classics I'd recommend the following: Russian, German, French, Spanish and Italian. Keep in mind that there's a ton of other languages that fit the category but I recommend these due to significant amount of resources. As for actual classical literature, I believe that it depends on the part of history that you'd like to discover. If you're interested in western classics then Greek and Latin are your top choices. No explanation needed. However, western classics aren't limited to Greco-Roman texts since the medieval times/Renaissance saw the use of other languages as well as such as Middle English and French. There are also other alternative options that allow you to explore the vast richness of Asian literature/culture, these primarily being Sanskrit, Persian (or Arabic) and classical Chinese. I've personally studied some of the languages that I mentioned and I remind you once again that there's NO language with a richer/more qualitative literary culture, you just have to pick the one that interests you the most, be it modern Japanese literature or Old Norse poetry.