How do you say you are ugly in Chinese?

你真丑 [nǐ zhēn chǒu] or 你真难看 [nǐ zhēn nán kàn]

Note: It's an offence to say so though you may be telling the truth. If you have to make a comment, better use 'you're not pretty' instead, which in Chinese is 你并不漂亮 [nǐ bìng bú piào liang].