How to identify lewis acids and bases
You need to draw the structure and identify the most acid and most basic site(s).
Notice how I didn't say "the most acid molecule".
Boron tribromide ($ \ce{BBr3} $) contains both a super-strong acidic site (the trivalent boron atom) and potentially basic sites (the lone pairs of the three bromine atoms).

Whether a molecule (e.g. $ \ce{H2S} $) will act as an acid or as a base depends on the reagents.