10. Akkadian

9. Biblical Hebrew

8. Coptic

7. Aramaic

6. Middle English

5. Sanskrit

4. Ancient Egyptian

3. Old Norse

2. Latin

1. Ancient Greek

Just because no one speaks them anymore doesn't mean they aren't TOTALLY BADASS.
When: 2800 BCE to 500 CE (all dates approximate)
What you can read in it: The lingua franca of ancient Mesopotamia, Akkadian uses the same cuneiform alphabet as Sumerian. The Epic of Gilgamesh, the Enuma Elish creation myth and other texts were composed in Akkadian, which has grammar similar to classical Arabic.
Pros to learning it: How impressed will people be when you can read those tiny little wedges? I'd be impressed.
Cons to learning it: No one really speaks it anymore.
When: 900 CE - 70 CE
What you can read in it: The Hebrew Bible, also known as the Old Testament, which was also later translated into ancient Greek (and then called the Septuagint.)
Pros to learning it: You can learn to point vowels, which is cool. Also, biblical Hebrew shares a surprising number of similarities with modern spoken Hebrew, including much of the alphabet.
Cons to learning it: No one really speaks it anymore.
When: 100 CE- 1600 CE
What you can read in it: The literature of the early Christian church, including the Nag Hammadi library, which contains the famous Gnostic gospels.
Pros to learning it: It's basically the Egyptian language written using the Greek alphabet, which is pretty cool.
Cons to learning it: No one really speaks it anymore. (Everyone switched to Arabic.
When: 700 BCE - 600 CE
What you can read in it: The vernacular of Second Temple Israel (539 BC - 70 CE) and the lingua franca of much of the Near East for centuries, Aramaic is commonly identified as the language of Jesus. Much of the Talmud is written in it, as well as parts of the biblical books of Daniel and Ezra.
Pros to learning it: It's not that much different from biblical Hebrew, so you can kill two birds with one stone. Also, you can pretend you're talking to Jesus, if that interests you.
Cons to learning it: No one really speaks it anymore, save for a few modern Aramaic communities.
When: 1200 - 1470 CE
What you can read in it: Chaucer, Wycliffe's Bible and "A Gest of Robyn Hode," which is one of the earliest tales of Robin Hood.
Pros to learning it: It's the foundation of modern English, which is cool. Also, Robin Hood!
Cons to learning it: No one really speaks it anymore.
When: 1500 BCE - modern times (as a liturgical language)
What you can read in it: The Vedas, a large body of sacred texts. Sanskrit was the lingua franca of the Indian subcontinent for three millennia and has a 49-letter alphabet.
Pros to learning it: Sanskrit provided the foundational texts for Hinduism, Buddhism, and Jainism.
Cons to learning it: No one really speaks it anymore, save for priests and some village populations.
When:: 3400 BCE - 600 BCE
What you can read in it: Anything extant carved into stone in ancient Egyptian artifacts and landmarks; papyri from various Books of the Dead.
Pros to learning it: It's fucking HIEROGLYPHICS.
Cons to learning it: No one really speaks it anymore.
When: 700 CE - 1300 CE
What you can read in it: The Eddas, a series of old Icelandic myths, and a bunch of runes and shit. The language of the Vikings, Old Norse was spoken in Scandinavia, the Faroe Islands, Iceland, Greenland, and in parts of Russia, France and the British Isles, and is sort of the predecessor to modern Icelandic.
Pros to learning it: You can pretend you're a Viking.
Cons to learning it: No one really speaks it anymore.
When: 800 BCE - the Renaissance. The so-called "golden" and "silver" ages of classical Latin spanned 75 BCE to the third century, and eventually the language transitioned into late and medieval Latin in later centuries.
Who wrote in it: Cicero, Julius Caesar, Cato, Catullus, Vergil, Ovid, Marcus Aurelius, Seneca, Augustine, Thomas Aquinas.
Pros to learning it: The Ryan Gosling of Dead Languages: it's popular and pretty sexy.
Cons to learning it: No one really speaks it anymore. (Except in a few diehard "Living Latin" communities, and at the Vatican, obvi.)
When: 800BCE - 300CE
Who wrote in it: Socrates, Plato, Aristotle, Homer (in a dialect called Ionic), Herodotus, Euripides, Aristophanes, basically everyone. The New Testament was written in a watered down version of ancient Greek called Koine.
Pros to learning it: You'll get an intensely larger vocabulary and learn to read a really badass alphabet. Plus, Aristophanes wrote about sex a lot.
Cons to learning it: No one really speaks it anymore.