the latin language
Latin seems to be the bane of many budding classicists in high school. It's an important language, though, both for historical research and for impressing people at parties and mixers. Also, it has a large number of immensely interesting texts.
In my early Latin days, I was told that between Latin and Greek, Latin was the easier to learn, but the more difficult to read at an advanced level. I have found this to be the case. Reading advanced Latin texts can be very hard, but it's also a lot of fun.