Letters from a related field

[Cesare]

Currently, I am getting 2 letters (one very well-known - the type of person people name drop without actually knowing, both with tenure) from my field/sub-field and one from a closely related field (however, I spent the least amount of time with him, but I do know he will write a good letter - i.e. nothing damaging, but warm and encouraging letter for graduate studies). For my third letter, should I try getting a letter from a Professor who I took several classes with, but probably doesn't remember me because he is literally the most forgetful person ever? 

[fuzzylogician]

I'm confused. You're getting one strong letter from someone in your field, one letter from a very famous person in your field, and then you need to choose between two "OK" letters:

- Someone in a related field who will write a "nothing damaging, but warm and encouraging letter for graduate studies."

- Someone in your field who you took some classes with and might not even remember you. 

I'm not sure what you expect the first professor to write exactly (is it just a "did well in class" or something more?) -- but how is getting a "did well in class" letter from someone you think doesn't know who you are a good option? Seems to me that if you can have a stronger letter from someone in a related field, that's a better choice. If you're basically choosing between two "did well in class" letters and the only difference is in the identity of the writer, you should find out which one is better known in your field and choose them.