If you're referring to a family:
* The Sullivans (This is the most common and formal way to refer to a family)
* The Sullivan family (This is also common and emphasizes the "family" aspect)
If you're referring to a business:
* Sullivan's (This is often used for businesses, particularly restaurants)
* The Sullivan's (This is less common but still acceptable)
If you're referring to a specific place or event:
* The Sullivans' house (This shows possession)
* The Sullivans' wedding (This shows possession)
Other possibilities:
* The Sullivans (the family)
* The Sullivans (the company)
Ultimately, the best way to write it depends on the specific situation and the desired tone.