Instead of typing up a whole essay I decided to Youtube my opinion on the reasons why I think it was a dream.
I think it was a dream at the end. My main four reasons are the first Architect, Cobb explaining the rules, his wife Mal, and the crazy Black dude smacking people in their sleep.
Here are other great threads as to why it might be a dream:
Was an Inception actually pulled on Cobb?
Did it end with a dream or was that Reality?
This was the best argument to why it is reality that I have seen so far:
Great articles trying to explain the movie: