Great question Skepcop. plus, I loves me some bigfoots.
I would go with #3 for sure.
If I was really sure that I had seen a bigfoot, I suppose that I would have no choice to believe the evidence of my eyes. I am not sure that anyone can ever be 100% positive about seeing something like that, but in your scenario it looks like it is pretty ironclad. Still, I wonder how I could be so sure? It doesn't sound like I saw it for very long. Like most people, I don't have a very good idea of what a bigfoot really looks like. How do you identify something you have never seen before? In a moment of startled surprise, is it possible to take in enough detail to say for sure that I wasn't seeing a costume or a bear? Just wondering. Chances are also good that the more time that passes, the more my memory of the event will degrade, allowing for further errors.
That said, I wouldn't expect anyone else to take my story as factual evidence. There is no reason for others to take my word for it. My story would justly become one of thousands of incredible and not to convincing tales told around the campfire. It would have been a golden opportunity lost forever. Curse my overly complicated camera bag!
However, I don't think your scenario covers every option. If I ran into a giant hairy biped, There is a good chance that he would leave some physical evidence. Rather than just leaving the area empty handed, I would look for something tangible I could take with me. Hair, a peice of gnawed on bark, or something like that to back up my claims.
That's my take.