I am a bit stumped about who to ask to write a letter for me. I have two pretty easy options, but it is the third that is giving me trouble. My research advisor is an obvious choice. Also, a professor who I have worked for as a TA for for semesters also seems like a good choice. But what should I do about the third? Is it okay to get a letter from a professor who taught me, in whose class I did well? I mean, it fills the requirement, but it doesn't seem very strong? Where do others go for there letters?