The best teacher for your LOR is going to be the teacher who knows you well and is willing to give you glowing reviews!!!
Bonus if that teacher is from a class that also relates to your planned major.
Most people choose a Junior year teacher because they have known them for over a year and their memory of you if still fresh and relevant.
Boulder requires 1 LOR- but you can always submit 1-2 more if you have teachers/councilors that can help add to your overall application in a positive way.