Definitely agree with what nebit said. One of the main reasons I took Spanish is because I'm going into medicine/field research and it's a super useful language to know, especially in the States. My other main reason was because my mom knows a ton. She has both her degrees in Spanish and was a high school Spanish teacher before I was born and an interpreter in a hospital for a while after that. She's basically my tutor lol. That, and Mexican/Puerto Rican Spanish has a ton of cognates. You can pretty much sub out any noun you don't know and you'll get by easily. But, Puerto Rican Spanish especially goes at 1000 mph so you definitely have to be a good listener.
If you're not exactly looking into a field where you'll be meeting tons new people on a daily basis (so some sort of lab or desk job) Latin can give you a massive boost on your SAT score. I've only taken the PSAT so far but word origins will really get you far in the reading section. If you have extra time in your schedule and can take a second language at least taking a year of Latin can do wonders. It's also super helpful for biology, especially anatomy--my first lesson in A&P I was just a massive list of prefixes and suffixes, the vast majority of which were Latin.
But, I can also see tons of benefits in learning French or German. It definitely opens you up to live in more of Europe. With Spanish you only really have Spain and even then there's a billion other related languages if you really wanted to know your way around.
Greek is a little more eh. It's not really used much outside of Greece and Cyprus, and technically some letters are used in STEM. But, if you particularly wanted to go and study Greek history or something I could see its use.
I can read and write many languages but am too shy/overcome with anxiety to actually speak them! For this reason, German was the easiest for me, accent-wise, and is also very very logical. 
