Easily the best one due to simplicity + action

The first film was too low budget and small scale to convey the actual Purge.

The third film was too bogged down in The Purge mythos, which is really not that interesting.

The second film is basically The Warriors, with a great lead actor and snazzy action set pieces. The Purge storyline is minimal until they get captured by the elite families, in the weakest part of the film.