Speaking as someone who recently played both games (and Hexen II) before these came out: Heretic is the best Doom title. It finally nailed the aesthetic that Doom was going for but never really met, while staying more or less true to the overall formula. I think starting with Hexen things started to get a bit too experimental, as I found each successive game less fun overall (mainly due to all the backtracking and trying to figure out what to do next, and not in a good way).