My money is on 'Taker.
As far as the rumors on it go...:
Sting - He won't go to the WWE and there is no logical reason for him to go there. He's also quite happy in TNA.
Jericho - Promos are too dark and evil to be him. Unless it's a "spoof" of sorts where we get the swerve and we get funny babyface Jericho back instead of the suit wearing boring heel Jericho. Possible, but I'll stick with it being 'Taker.
Vince/Shane/Steph - This is possible too, but I seriously doubt it. Shane is the biggest question mark since he's overseas working on expanding a pay-per-view company. Could sub out Shane for Triple H, but I'm just not sure that will happen. Promos are still too dark. Vince tried to be the "higher power for 'Taker quite some time ago and the storyline just didn't fit McMahon as actually being that evil. Bad? Sure. Mean? Sure. The Purity of Evil? No. So I doubt this is it, but the addition of the little girl made me wonder for the very first time since they started running these promos. Was no question up to this point and still really isn't a question in my mind, but that was an interesting element to add.
HBK - Retired, filming a hunting show, enjoying life. Plus he wouldn't be the type to wrestle after being "retired" like practically everyone else is/did after being "retired" (Flair/Foley/etc). Not to mention Shawn was never really a "dark" character. Had a mean streak for sure, but big difference in that and these promos. Can't believe I typed all that about it not being HBK, it obviously isn't, but whatever.
Austin - He isn't really gone-gone and I'm sure there will be a second season of Tough Enough, especially now that The Network will *officially* debut April 1st (same day as Wrestlemania). So that's really unlikely.
Kane - Already has his own promos running.
Brodus Clay - Ditto. And he was supposed to debut weeks ago but it keeps getting delayed due to time constraints.
After typing all of that out, here's a scenario:
We get back the Ayeatollah of Rock 'N Rolla Funny Babyface Jericho who comes out to huge cheers and cuts a promo about how he had everyone fooled into thinking it was someone evil returning, and then the gong hits and 'Taker shows him who those promos were really for.
Would give 'Taker a warm up with a solid veteran before taking on whoever at Wrestlemania. Speaking of that, it appears that Wade Barrett will be the target for 'Mania with 'Taker, unless that's what the return of the mask with Kane is all about... not sure Kane is ready just yet to retire, but a 3rd match against 'Taker at 'Mania would probably signal the end for the big guy. Regardless, it seems this heel run Barret is on is setting him up for 'Taker at 'Mania.