If you're thinking about someone being focused and engaged:
* Inattentive
* Distracted
* Absentminded
* Unmindful
* Careless
* Neglectful
If you're thinking about someone being considerate and responsive:
* Inconsiderate
* Dismissive
* Rude
* Unresponsive
* Indifferent
If you're thinking about someone being observant and perceptive:
* Oblivious
* Unaware
* Blind
* Unseeing
The best opposite word depends on the specific meaning you want to convey.