What's better (or has the better/bigger impact)?: Revealing extremely important details about the protagonist early on (ex: people find out that the protagonist is a royal or some extremely powerful drug lord), and the rest of the story focuses on his experiences and adventures whilst he deals with his status. Or save that for the climax for the "AHA!" moment where everyone's jaws drops and is speechless, etc.
The "AHAH" reveal at the end might work for a short story--the main character being a powerful drug lord, for example, but probably not in a novel. Some of it depends on the POV used. It also depends on if you, as the author, have to do all sorts of acrobats to avoid revealing what would be otherwise obvious...avoiding it in dialogue, thoughts, character actions, responses, etc. Finally, it would depend on the purpose for including such a 'reveal' which would be related to the overall purpose behind writing the story.
Revealing things straight away or at the end - both are viable options, I don't think you can say which one is better, but they will have different efect on the story. Keeping it for the end might have a stronger impact, but it's also trickier to handle so as the readers do not feel cheated. An important fact like those you mentioned should probably influence the storyline, so you need to make sure that the story doesn't contradict itself and the revelation still feels believable in the end. You might want to foreshadow it, suggest that there's more to the character than we immediately see.
I'm with TWErvin2, big shocking reveals are probably best kept for short stories unless we're talking horror or mystery novels. The trouble with the big reveal at the end is you're keeping things from the reader that he should probably know or have a really good reason why he doesn't. It becomes very cat and mouse and it can actually backfire if it isn't handled right. The reader can feel cheated that the clues weren't properly handled or dished out. Plus it tends to feel gimmicky if it becomes the whole point of the story. You can get so side tracked in withholding 'evidence' or a true nature, relationship or job that the scenes can feel shadowy and subverted. But then again it depends on what you're doing, your genre, and the reveal. I've had this problem with one of my novels. I thought about and tried switching povs to help resolve the issue. I think the issue is more about what I want the reader to focus on. I think the reader knowing ahead of time can add more drama to the scenes. I remember Hitchcock once said there is no tension in a little boy carrying a package and then it blows up. The tension is when you know the package is a bomb.
If he's secretly a drug lord, but working in public as a politician or fried chicken restaurant owner, then revealing that near the end could work.
