|TEACHERS ARE UNDERPAID|
Ok. So, UC's do NOT recommend LOR. So, if you're like me and only applying to UC's you can read another mini-guide. You're probably wondering how I know so much about LOR, well, not only do I have a rich bank of probably days worth of YouTube videos about students and their own experience, but I also asked for some! Yep! I had to muster the courage only to never send them :). But a majority of other schools do require them.
CHOOSING A TEACHER
Choosing a teacher should follow the basic guidelines of who you've known the longest and the one's that know YOU best. I chose my science teacher who wrote an amazing essay for me (Since I technically never submitted my LOR to these schools, she did show me before submitting them) even though I didn't talk to her much because I followed this rule from Youtube (watch the whole video) called the Halo Effect, works all the time.
Then, since I had literally no other teacher, I had to ask my principal who taught me Honors English 11 and 12 and she loved me to. In conclusion, YOU know best who is optimal to write your LOR. Now, if you have a plethora of options there are some general rules you should follow.
In short, if you are entering STEM.
Let's say the school requires 3 LOR. You want 2 from STEM, and if you're like me and took principally DE you have to suck it up and ask a professor. Try your absolute hardest to get classes with the same professor if you get along with them and they seem to like you (you know what I mean). Then, typically you would want a third from a humanities teacher because it shows colleges you know your stuff. No seriously, by adding a humanities teacher you are able to show, not that you are well rounded, but that you are well-equipped to succeed in all classes that you are required to take at the college (because you will be taking history, art, philosophy, etc. in college, again.)
Here I wouldn't recommend getting one from a STEM teacher only because humanities majors generally don't need to express adequacy in STEM classes. So "YOU know best who is optimal to write your LOR" applies here.