General Opposites:
* Careful: This is the most common and direct antonym.
* Attentive: This suggests paying close attention to detail.
* Mindful: This implies being aware of one's actions and their consequences.
* Thoughtful: This suggests considering the impact of one's actions on others.
* Diligent: This emphasizes hard work and meticulousness.
Specific Opposites:
* Conscientious: This describes someone who takes responsibility and acts with integrity.
* Meticulous: This implies extreme attention to detail and accuracy.
* Precise: This focuses on accuracy and correctness.
* Thorough: This indicates completeness and attention to every aspect.
The best antonym for "careless" will depend on the specific situation and the intended meaning.