Of the three you mentioned, arimidex is the weakest, but I believe that makes it the best. You can better adjust/more accurately the doses. Aromasin and letrozole are much stronger and should be used only at high mg/wk test. The stronger AIs also have more potential to adversely affect your lipid profile.
Letrozole (and I believe aromasin too) have relatively long half-lives and need to be front loaded (or should be)
Use Adex.