Red John has to be a main character of the series, otherwise the whole thing would make no sense. Of course there are some clues about Ellis Mars, Partridge, ....., but the usual suspects do not appear in enough episodes to make sure that the average TV-watcher know them. If at the end some guy who emerged in the first season turns out to be Red John, half of the audience do not know him and will be disappointed. 
We can exclude Lisbon and VanPelt, both are women, and probably also Rigsby (sry he is simply to dumb). And would it be surprising if its Cho? Not really...
The only possibility is Jane!! (And forget about the evidences, voice checks, scenes were Jane and Red John meet, ....,every clue can be pointed out in hundred of different ways)
 Its to crazy/unlikely to be true especially because I think he is a very popular character, how ever that could be the opportunity for Bruno Heller to make TV history!!!
AND HERE NOW THE ULTIMATE PROOVE: RED JOHN IS THE OPPOSITE OF PATRICK JANE!!!!
JANE TURNS TO THE MALE FORM OF THE NAME : JOHN
PATRICK, THINK ABOUT ST. PATRICKS DAY: EVERYTHING IS GREEN, AND THE OPPOSITE OF GREEN IS ....... RED!!