Who to ask
A teacher who has seen you struggle with something and get better at it, ideally in a core academic subject and ideally recently — junior year teachers are usually the strongest choice because they saw you most recently doing the hardest work.
Not automatically the class where you got the highest grade. A teacher whose class you found easy often has less to say than one who watched you work at something. "She was excellent" is a weaker letter than "he rewrote this essay three times and the third one changed my mind".
If a college asks for teachers from different subject areas, respect that — it is asked for a reason.