In a software that follows any specific SDLC, who can be considered the mostappropriate resource to catch the defect.

Software development life cycle has various stages and each stage has
different experts or resources involved starting with product owner, business  analyst, designers, developers and testers. Every of them somewhere or the  others perform testing to identify bugs at their own level. The developer does  unit testing, testers are officially there to test the application and identify bugs  however product owner and business analysts is the best person who can  identify the bugs in accordance with the business requirements.