Hi, It's hard to say. it's differ from town to town, region to region and so many other factors. But i guess Ensha'allah (ان شا الله) with Persian pronunciation Ishalla ( ایشالله ) is the most used idiom or cliche. Although it look likes a word, but it is a sentence(sometimes). It means: if God wants.

actually it`s impossible to choose one or two idioms so i write for you some of the common ones: mesle khar kar kardan:working too hard. rodederazi kardan:bending someone`s ear ba dom gerdo shekastan:feeling on the top of the world. dast o pache shodan:lost someone`s head