Welcoming and unfriendly are the two opposite antonyms of the word "un-welcoming". When someone is welcoming, they greet others warmly and invite them in with open arms. Their demeanor is friendly, hospitable, accommodating and inviting. On the contrary, unfriendly is being distant, cold, and impolite. A place or an environment that is unfriendly makes someone feel unwanted and uncomfortable. Hence, if one wants to convey the message of being hospitable and friendly, welcoming is the perfect antonym of "un-welcoming". And if one wants to describe an atmosphere that makes them feel unwelcome or uncomfortable, unfriendly is the fitting antonym to use.