Acording to Heller Red John is one of 7 people on that list.

1.Bret Stiles
Never killed a person (at least not proved), would rather have someone else do it.
Red John enjoys to kill, Bret Stiles not (or he probably would have killed his victims himself).

2.Gale Bertram
Was already suspected of being an accomplice of RJ.So it would be boring from a narrative view to have him as RJ.

3.Ray Haffner
Member of Visualize. Fits the profile and is already an established character on the show.

4.Reede Smith
Appears in Season 5. You don't know a lot about him, so why is he on the list?

5.Robert Kirkland
Appears out of nowhere in Season 5. More of a distraction, but he's definitly "evil".

6.Sheriff Thomas McAllister
Doesn't seem to have the intelligence and looks like a random choice to me. But he will return in Season 6.

7.Brett Partridge
Most theories say it's him, but I've got a feeling that Heller wouldn't make it that easy.

My top three are: Ray Haffner, Brett Patridge and Sheriff Thomas McAllister.