• How have you been? 
(this can be formal or informal)
• What have you been up to lately?
• How’s it going? (informal)
• How are things? 
(informal)
• How’s life?
• What’s new?
• How ya doin’? (informal) ya is actually you and doin is actually short for doing
• How’s your family?
• What’s up? (informal) this is slang which means it’s very informal and most common in speech rather than in writing

Which out of the above list do you use most?