General obligation:
* Should - This is the most common synonym, and often interchangeable with "ought."
* Must - This implies a stronger obligation or necessity.
* Have to - Similar to "must," but can sometimes be used for external pressures rather than moral obligation.
* Be supposed to - This emphasizes an expectation or rule.
* Be obliged to - This focuses on a formal or legal obligation.
* Be required to - Similar to "be obliged to," but emphasizing a rule or regulation.
Moral obligation:
* Should
* Have a duty to
* Be morally bound to
Personal opinion/advice:
* Should
* Would be best to
* It would be advisable to
The best synonym for "ought" will depend on the specific situation and the desired level of formality or strength of obligation.