Fill in the gap with "a", "an", "some" or "any". There is ______ comfortable armchair in the living room.

The sentence is a statement about the existence of a single, non-specific comfortable armchair. Therefore, the indefinite article "a" is appropriate. The word "comfortable" starts with a consonant sound, so "a" is used instead of "an". "Some" and "any" are typically used for plural or uncountable nouns, or in questions and negative statements, which is not the case here. Thus, the correct word to fill the gap is "a".

Answer: a