Well if you see, all the incarnations of Lord Vishnu are considered as powerful. SriNarasimha is as strong as SriRama or SriParshurama or SriKrishna. Yes, you may say why Lord Narasimha is considered as the most fierce avatara of Lord Vishnu.
In terms of power, knowledge, beauty, charm, opulence, valor all the avataras are the same, but if you talk about Narasimha avatara, then it was taken for destroying the evil forces and protecting devotees, so moreover he is the protector form of Lord Vishnu. If we see other avatars, yes they were also taken for evil destruction, but Narasimhavatara was primarily taken for immediate protection of a devotee, which simultaneously required destruction of evil. Even when the devotees are scared, or for removal of any sorts of black magic or anything like that, we go in the refuge of Narasimha roopi Narayana because he is ugra only for the evil and is shaantamoorti for the good people.
Hence in a nutshell, there is no difference between any avataras of Vishnu and all are the same and equally powerful. So, it is not like any one avatara is considered as the most powerful. All the avataras are powerful. Yes, Narasimhavatara is considered as fiercest avatara of Vishnu for evil destruction but he too is fierce for the evil while he is calm for a devotee.
Hope this helps!
Jay SriRam!
ShriKrishnarpanamastu!