By "any help" I mean that by another person, any type of guide, any information from the internet, etc. Just you and your wits and skills.
For me, I'd have to say Super Ghouls and Ghosts for the SNES. I was 11 when the game released and I played the hell out of it for months on end. I figured out how to get certain chests to spawn, memorized enemy spawns, realized which weapons were the best for every area and boss, was able to do no damage runs, and of course got the true ending. Probably stuff I wouldn't be able to do anymore, but I feel it was a great accomplishment in my video gaming life.
Without truth, there is nothing.