Hi there Mahsa!
for dictionary I usually use Oxford Advanced Learner's. I have both the software and the book. software comes more handy compared to the book, you know, it's faster.
I have also Cambridge Advanced Dictionary, which is good, but Oxford's example sentences for each word is richer!
If you need a good English to Persian Dictionary, especially if you like translation, it's a good idea to use Farhange Moasere Hezare, usually known as Hezare Dictionary by Dr. HaghShenas.
but, about a good one for your mobile phone, I need to know your device's operating system, I mean is it Android or Symbian?
If you own a Symbian phone, as mine, there are good series of MSDict. but you need to kind of hack you phone which I can give you some links and websites to do that before installing it. and the good point is that, these series have audio pronunciation, but unfortunately it didn't work on mine!
you know I have the file, but it's around 300 Mb, and I can't send you that. I got from a friend, and I don't access the link. but the name is :
Audio Advanced Learner's Dictionary 3rd Edition
and also this one is good:
Collins English Dictionary
and
Concise Oxford English Dictionary
now if you have a Android phone, just get the Bazar application and there you can find hundreds of really useful dictionaries. there are a lot of them for Android phones. you can find them in a simple search in Google as well, then download and install them, you yourself can easily find if it is good or not.
hope it helped!
