Formal:
* 您好，您今天好吗？ (Nín hǎo, nín jīntiān hǎo ma?) - This is the most formal way, suitable for addressing someone older or in a position of authority.
Informal:
* 你好，今天过得怎么样？ (Nǐ hǎo, jīntiān guò de zěnmeyàng?) - This is a more casual way to say hello and ask how someone's day is going.
* 嗨，今天怎么样？ (Hài, jīntiān zěnmeyàng?) - This is even more casual and friendly.
Other options:
* 早上好，您今天怎么样？ (Zǎoshang hǎo, nín jīntiān zěnmeyàng?) - Good morning, how are you today? (Use this in the morning)
* 下午好，您今天怎么样？ (Xiàwǔ hǎo, nín jīntiān zěnmeyàng?) - Good afternoon, how are you today? (Use this in the afternoon)
* 晚上好，您今天怎么样？ (Wǎnshang hǎo, nín jīntiān zěnmeyàng?) - Good evening, how are you today? (Use this in the evening)
Remember:
* The use of "您" (nín) is a sign of respect and is generally used when addressing someone older or in a position of authority.
* The tone of voice and facial expressions also play a role in conveying the level of formality.
* The best way to say "Hi, how are you today?" in Chinese depends on the specific situation and your relationship with the person you are talking to.