Horizon, but even that was with the Amazon discount. Before that...hmmm...Diablo III: Ultimate Evil Edition...I think.
Very few games strike me as "must play" nowadays. Not only do I save money, but I get to play better, patched versions of the games by waiting. Meanwhile my backlog has grown so big that I've got major AAA titles sitting there just waiting to be play. For example, I'm playing Doom for the first time right now. So 3 months ago I had the option of paying full price to play the iffy Mass Effect: Andromeda or I could have fired up my $30 copy of Doom. Needless to say Andromeda had little chance.