Formal:
* Ineffectual: Lacking in power or effectiveness.
* Fruitless: Producing no result.
* Futile: Pointless or hopeless.
* Worthless: Having no value or use.
* Pointless: Having no purpose or significance.
* Unproductive: Not yielding or producing anything useful.
Informal:
* No good: Not useful or satisfactory.
* Rubbish: Worthless or useless.
* Crap: (Informal slang) Worthless or of low quality.
* Good for nothing: Completely useless.
* Waste of time: Something that is unproductive or pointless.
* Dead weight: A person or thing that is useless and burdensome.
More specific synonyms:
* Inefficient: Not using resources well.
* Incompetent: Lacking the necessary skills or abilities.
* Ineffective: Not producing the desired result.
* Redundant: Unnecessary or superfluous.
The best synonym for "useless" depends on the specific context and the tone you want to convey.