Is #"NH"_4^(+)# an acid or a base?
More often than not, an acid. It donates a proton by accepting two electrons. For instance:
Ammonium is the acid, donating the proton to water (Bronsted-Lowry definition) by accepting two electrons (Lewis definition) from water's oxygen.
Water is the base, donating two electrons (Lewis definition) from oxygen to accept one proton (Bronsted-Lowry definition) from ammonium.