A lot of great theories and speculation on here but my main suspect is Sheriff Tom McAllister.

A lot of great points are made as to why it is plausible to be him with Theory #3066 on this site. I won't rehash those valid points but here is a new one to consider:

In episode 2 of Season 1 where McAllister is featured he appears to be looking to prey on Grace Van Pelt before Rigsby intervenes. He acts shocked that they would think he would be a killer. In the words of a famous English playwright "Me thinks doth protest too much!"

The seed has already been planted with McAllister.