John Cena vs. Randy Orton: Viper Wins WWE World Heavyweight Title

In what was one of the year's most anticipated matches, Cena's World Heavyweight Championship and Orton's WWE Championship were merged to determine the one true champion in WWE. Somewhat surprisingly, the WWE brass decided that Orton was the right man to carry the torch.
Orton became Triple H's handpicked face of the company back at SummerSlam as Triple Pedigreed Daniel Bryan after he beat Cena for the WWE Championship. This allowed Orton to successfully cash in his Money in the Bank contract.

The Viper continued to feud with Bryan after that, and he ultimately exited the angle as WWE champion thanks to some help from Shawn Michaels at Hell in a Cell. Orton then moved into a brief feud with Big Show, which culminated in Orton beating the giant at Survivor Series.
It was unclear what direction he would go in after Survivor Series, but WWE clearly felt as though it needed to do something big. Survivor Series was terribly booked and built, and it can certainly be argued that it was one of the worst WWE pay-per-views in recent memory.
With that in mind, the unification bombshell was dropped the next night on Raw, and it has been a whirlwind ever since. It was initially unclear how it would be handled, as there was some thought that Cena and Orton would either grab the same titles that they left with or even trade titles, but Triple H made it abundantly clear that unification would occur.

That, in and of itself, probably sold plenty of pay-per-views since fans knew that it would be a truly unique moment. The build was furthered by the go-home edition of Raw too as an ascension ceremony featuring several former WWE and World Heavyweight champions took place to close the show.
Both Cena and Orton did a nice job on the mic before things deteriorated into a brawl. A number of things happened such as CM Punk attacking Orton, Triple H attacking Punk, Michaels hitting Punk with Sweet Chin Music, Bryan hitting HBK with a running knee, Orton trying to hit Bryan with an RKO only to inadvertently knock over Stephanie McMahon and finally Triple H Pedigreeing Orton.

Now that Orton is the first Unified champion since Brock Lesnar, it will be interesting to see how his reign is handled. He obviously has to defend his title against someone at Royal Rumble, and it seems likely that it will be against Cena since he is entitled to a rematch.
Beyond that, however, it is anyone's guess how it will play out. If the plan is for Orton to carry the title into WrestleMania XXX at the very least, then he'll be facing the Royal Rumble winner.
As of right now, Bryan and CM Punk are probably co-favorites, and while Orton has already faced both of them in some big matches, there is no question that Orton could put on a great match with either of them.
The big question relates to whether or not Orton vs. Bryan or Orton vs. Punk would be worthy of the WrestleMania main event, especially since Orton and Bryan just got done feuding.
It's likely that a large contingent of fans isn't particularly happy to see Orton in this position after a lackluster run as WWE champion, but it certainly opens up plenty of possibilities moving forward.