Is it appropriate for me to get a recommendation letter from a teacher whom I TA for, but whose class I have not taken in 3 years? I am a teaching assistant for a biology teacher. When I took biology 3 years ago, she was my teacher. There is another teacher who she works EXTREMELY closely with, and there is essentially no differentiation between the two teachers regarding what I help out with, and I am close with both of them. 

Is it appropriate for me to get a letter of recommendation from one of them even though I took the class as a freshman and am now a junior teaching assistant? For any other freshman teachers, I would not consider getting a letter of recommendation; however, I have gone far above and beyond the normal duties of a teaching assistant -- namely designing and implementing labs, grading, and even teaching the class as a substitute (there was an official proxy but I did the teaching). Humbly, I am quite appreciated, trusted and I have no doubt that I would receive quite a stunning recommendation letter. I would love to have a recommendation from one of the two marvellous teachers that I TA for, but I cannot decide which one -- the one whom the position is designated to? I feel there is potential for the other to be offended, and I would much prefer that that didn't happen. A selfish part of me wants the 'better' writer to write my letter, although I don't know who that is and I would prefer not to think of it that way (though a part of me suspects that is it the teacher to whom my position is not officially designated). 

Just to clarify, I am in high school, and the recommendation letter is for undergrad/combined university programs. 

Is it appropriate for me to get a letter of recommendation from one of these teachers? If so, how should I choose?

Edit: one of the teachers is also the advisor for a club that I am president of.
yes, anyone who has supervised your work is suitable to be a recommender. You should solicit recommends who can give you the best recommendation and also whose professional status is most impressive.