Storyline wise I would go with the first Saw or the second one, we saw some fightback to Jugsaw by Donnie Wahlberg, but when it come to the most gruesome explicit puzzle in the entire Saw series, it would have to go to the Twister Rack in Saw 3. This puzzle was disturbing on massive levels. Never saw it in theater but I just can't imagine at least 70% of the people in there shutting their eyes away for that scene.