It depends obviously on your intended use for the language.
Ancient Greek is presumably for reading the classics in the original and impressing some people.
The scientific language of foremost usefulness is English, and after that French and German for reading classic works of the 19th century.
A few 19th century works of science are in Latin, from a wrong guided prediction of what the international language of science would become.
Italy is many people's favorite country to visit, and hence it would be useful to be able to speak to the locals in their own tongue.
Latin America offers a huge variety of destinations where Spanish is the chosen language, and there is a lot of wonderful literature in Spanish.
China is the one of the most populous countries, and with the current rapid decline of US capitalism, may become the most powerful economically in your lifetime, making Chinese a potentially useful language for business as well as travel.
The rhetorical question above reminded me of one i got from an otherwise intelligent parent, when I was trying to advocate teaching French to junior high children at a local private school: "who needs to know French?"
Only a person remarkably untraveled could ask such a question. Having visited France, Germany, Mexico, Spain, Holland, Norway, Denmark, Switzerland, Italy, Japan, Chile, and having friends who are Russian, Swedish, Bulgarian, Turkish, etc.. etc.., I have at times wished I knew all these languages.
I have learned more than a few words of 6 or 7 of them, and can accomplish some basic duties in French, Spanish, and German, and read some of these as well as Russian and Italian.
Even back when I was reasonably fluent in Mexican Spanish, I encountered whole villages of indigenous people in that country who could not understand a word I said in it, presumably indians.
I can say thank you to shopkeepers in Korean and for a time in Bulgarian, and goodbye in Mandarin, but it slips away if you don't use it, like math.
My Dutch, Portugese and Norwegian are pretty non existent though.