1. All PIs are musts. So ask your freshman and current PI absolutely. Same goes for the research supervisors for your current lab. There's 4 letters right there...
2. How did the research-oriented course go? Are you close with that prof? That's an option.
3. Foreign lang. prof sounds stronger than the English prof. Which one writes better? That does matter.
4. Other profs / outside recs
I agree with above, but you don't want to ask 10 profs if you're only gonna use 4 letters, you know? Just make sure you're actually considering using their letter if you ask them for one (don't waste their time). Go in the order I listed above when asking. Also make you have at least one non-science rec letter, a lot of schools require one.