It's McAllister.

=> Evidence #1: http://i.imgur.com/v98oWsz.png
=> Evidence #2: "He is Mar", could mean "He is McAllisteR" or "sHErIff thomaS McAllisteR" or even "He is McA".
=> Evidence #3: Waldo. He stays in the background and appears once a page. McAllister appeared in one case and stayed in the background for the rest of the time.
=> Evidence #4: "I'm surprised you didn't become lifelong friends from the moment you shook hands." They shook hands when they played rock, paper, scissor.

There's also some other things, like the fact that he was the only one with title on the list. I honestly didn't like the idea at first and even till last episode. Now I'm happy with basically anyone but Bertram. Talk about an anticlimactic reveal.