While a bit too much of a twist with the neighbors near the end, I actually liked the first Purge movie the best. Something about making bunkers, forts, or armoring the the houses up trying to keep crazy people out that just seemed more involving for me. It felt like the characters were more vulnerable and the sudden twist of the boyfriend trying to off the father was definitely shocking and the neighbors trying to be overly nice to once another for fear of rubbing someone the wrong way just slightly was intriguing. There was a true social underpinning behind everything amplified by a truly outrageous concept.
With that being said, I think the 2nd movie was ok. Frank Grillo was awesome in it and would've made a great Frank Castle. However, for the Purge franchise, I think the main thing the movie franchise needs to avoid is an unstoppable action hero as then the idea of a Purge becomes more of a backdrop to an action movie. The trailer for the 3rd makes me think this will probably be the case but the movie will still probably be worth checking out though.