Since I started studying Mandarin in 2008 I've come across many Chinese words, but few phrases and words have really caught my attention as much as 岂有此理! (qǐyǒucǐlǐ!). It is used as an idiom/expression and literally means "How can this be so?", but used an expression it means: outrageous, preposterous, absurd. It can be used as a separate expression or in a sentence such as:
他的女朋友很美丽，真是岂有此理！(tā de nǚpéngyou hěn měilí, zhēnshì qǐyǒucǐlǐ! - His girlfriend is really beautiful, it's outrageous)
Ok, that sentence is little tongue-in-cheek, but I just like how the phrase works. It also sounds really nice and rolls well of the lips: qǐyǒucǐlǐ! Another reason why I like this word, and part of why I remembered this phrase is because of 岂 (qǐ). The character reminds me of a person with a fancy crown on their head and is thus "outrageous" (said in a socialite Paris Hilton kind of way).
你有没有最喜欢的言辞？(nǐ yǒu méiyǒu zuì xǐ huān de yán cí - Do you have a favourite word/phrase?)