The original Kingdom Hearts. In 2003, I was hit with kind of a financial perfect storm, a combination of a legal battle, pay cuts at work, and medical issues. I went into debt big time, and I was doing everything I could just to keep lights on my apartment and food on the table. Pretty much sold every non-essential thing I owned. It took me about 2 years just to claw back to plain old-fashioned "not well off."
As I was finally starting to see some light at the end of the tunnel I randomly came across $20. I hadn't bought myself a video game in a very long time, so I hit GameStop and found a used copy of Kingdom Hearts (The PS2 was one of the only fun things I held on to, though I didn't have much to play on it).
For the next couple months I dived in as deeply as you possibly could into that game. I found every single treasure chest, Dalmatian, and any other secret there was. Fought every optional boss, made it to the end of the coliseum, you name it. Whatever that game offered I experienced.
Kingdom Hearts was very much a life preserver for me during an unhappy time, and to this day even as rough as the combat is, I can still play it in it immediately brings a smile to my face. The series has meant quite a bit to me over the years, at least it did up until KH3.... But hey, that still doesn't take away how much I love the first couple games.