Claudius

He is, first of all, not really the next in line for the throne.  Hamlet - the main character of the play - probably should have succeeded his dead father.  Secondly, he has married his brother's widow which, in the time of the play, was considered incest (odd, but true).  Third, he seems to drink too much and party too much when, in fact, there is some attack coming from Norway which he spends minimal time negotiating or worrying about.