General Opposite:
* Unfriendly is the most direct opposite, meaning not welcoming or pleasant.
Specific Opposites:
* Hostile: If you're thinking about aggression, hostile is a good opposite.
* Cold: If you're thinking about warmth and openness, cold is a good opposite.
* Aloof: If you're thinking about distance and detachment, aloof is a good opposite.
* Unwelcoming: If you're thinking about a lack of hospitality, unwelcoming is a good opposite.
The best word to use depends on the specific situation and what you want to emphasize.