My immediate thought was Undertaker because we've practically been conditioned to think any time a creepy promo comes on it has to be him. But there are a couple reasons why I think it can't be Undertaker and is indeed most likely Jericho.
1) The video said he's coming to "take back what is rightfully his"...What would the Undertaker have to come back to take? He only has the streak now and its not like he lost it or anything. Its already his. The other side of that is what everyone has already said, Jericho would be coming back to take back his "best in the world" moniker.
2) This seems painfully obvious and I'm surprised no one pointed this out yet. The 2nd day of 2012 is January 2nd. "Y2J." 2nd day, J for January. Too easy. That was part of the whole code thing last time, numbers and what not. I wouldn't be surprised if they went a similar route with his 2nd return.
3) We're all overlooking the simple fact that this was a subliminal, youtube only promo. It did not air on TV. If there was ever 1 superstar in the history of the WWE that would use a cryptic, online only ad for a return its Chris Jericho. Since he left he's always said if he does come back he would want to be difference once again. Putting cryptic messages that you have to go online to see the whole thing would be a new way to promote a superstar. That would be perfect for Jericho. Why would they hype the Undertaker's return through an online video? On youtube of all places, not even WWE.com. That doesn't fit the character.
Lastly and most importantly 5) Why on earth would the Undertaker come back the first week in January? Last time he didn't come back till the end of February and only wrestled at Mania. This would imply Undertaker would be wrestling at the Royal Rumble AND Elimination Chamber. I could see a return at Rumble as a surprise entrant then he disappears till right before Mania. His body barely made it through Mania last year. Undertaker doesn't need a 3 month buildup at this point in his career, 1 month would do it. 4-5 Raws, quick feud with someone, promote the streak. 3 months would be overkill. This doesn't necessarily prove its Jericho but it damn sure strengthens the idea that it's NOT Undertaker.
Oh by the way, it's not Kane. Just no. They wouldn't air 1 promo showing his face and mask then have a totally different one in a cryptic form. That makes no sense.