General:
* Defective
* Imperfect
* Flawed
* Erroneous
* Inaccurate
* Damaged
* Broken
* Defective
* Malfunctioning
* Substandard
More specific:
* For products: Defective, broken, malfunctioning, damaged, flawed, substandard, shoddy, inferior
* For reasoning or logic: Flawed, erroneous, inaccurate, illogical, unsound, invalid
* For behavior or actions: Erroneous, mistaken, wrong, misguided, flawed
Informal:
* Buggy (for software)
* Crooked (for something dishonest)
* Dodgy (for something suspicious)
* Off-kilter (for something out of balance)
The best synonym will depend on the specific context of your sentence.