Theory #1: Cobb went back to reality and was reunited with his children.
This theory is the one most viewers would acknowledge as the “most correct”. An optimistic interpretation, this theory is in my opinion the strongest, and is quite difficult to refute even though there are a few grey areas.
When Cobb woke up in the airplane, that was reality. In his dream, he successfully performed inception on Fischer. He also managed to convince a stuck-in-limbo Saito to honour the arrangement they had made which allowed him to pass through the US customs without a glitch. Accompanied by Miles, he returned home to his children. He spun his top and walked away, not even bothering to look at the outcome, which was a sign that he was confident it was reality.
The children have also aged (albeit only slightly) since Cobb was away for about a year. Nolan used older child actors for the final scene, backing up the claim that Cobb’s children did indeed age. They seemed to wear identical clothing but on closer observation, they were only wearing very similar clothes. Even if they wore the exact same clothes like they did in Cobb’s dream, it wouldn’t be much of a hoo-hah too. Why? Ask yourself this: How many times in a year do you wear the exact same clothes while at home? My guess would probably be at least twenty times. So my point is that coincidences do happen and they do not always surprise.
Everything that had happened in the film prior to Cobb waking up in the airplane could only have occurred if there was a solid platform for these occurrences to, well, occur. This solid platform was provided by the assurance that there was a reality, and that reality was the fact that Mal had killed herself, and Cobb was subsequently forced to leave his children. Without reality, there would be no motivation to talk about dreams, limbos, totems, extractions, inceptions etc. because these concepts were created based on the existence of the dream-reality bi-construct, or the dual existence of both dream and reality.
Another observation would be the wedding ring on Cobb’s finger. In the dream sequences, he wore the ring, but in reality he did not. In the final scene, he did not wear any ring. This meant that the ending was reality.
Last but not least, the final shot of the spinning top that wobbled ever so slightly before Nolan cut to black was an indication that it was reality. Why? In a dream, the top would spin smoothly and indefinitely. But in reality, it would wobble and fall. That wobble at the end (even if it was only a split-second) suggested that it was reality.