Which opening is most defensive as black? I am relatively a new chess player and been improving my strength in the opening for a while and would like to know which opening is the most defensive. 1.e5 seems a little too aggressive.
This is a subjective question, so I'll give a subjective answer. In my opinion, the Hippopotamus Defense is the most defensive opening. Here's a typical Hippopotamus structure - 

   [FEN "r2qk2r/pbpnnpbp/1p1pp1p1/8/2PPP3/2N1BN2/PP2BPPP/R2Q1RK1 w kq - 0 9"]


Black doesn't even move a pawn to the 4th rank! All of Black's minor pieces are developed on the second rank and Black keeps his pawns on the third rank. Essentially, Black plays a waiting game for a while, and tries to provoke White into doing something concrete and then Black reacts accordingly. 

P.S. Since you're a beginner, I would advise NOT to use this opening. Yes, it is the most defensive opening, but if you're afraid of playing 1...e5 then you ought to first correct that instead of trying to play something else! Even with 1...e5, Black can reach defensive setups.