Final Fantasy VII.
I was already a veteran gamer when I first bought it, but I had never tried a JRPG before. I had great fun, and would often abandon my save file and start over again for little to no reason.
Needless to say, it took me some time to get to Sephiroth using this approach, but I got there eventually. Cue an epic, 2 - day battle (I had to pause the game overnight to ask a friend for advice on how the first part of the battle worked) which concluded with Sephiroth mashing my team into lapskaus (traditional Norwegian stew).
I started over again. From disc 1. This time, it was personal. I bought a guidebook and kept at it until I had Knights of the Round and everybody's ultimate weapon. Then, I started grinding. Then I kept on grinding.
And then, Final Fantasy VIII went platinum.
I felt like a break from FFVII, so I bought VIII, played it for a while, and found it to be absurdly overcomplicated and mind-numbingly dull. (The characters were so bland and poorly defined, I actually had trouble telling them apart!)
So I quit, and forgot about the FF series until Final Fantasy IX came out. I bought it on day one, never thinking I'd actually finish it, but I knew that, being a Final Fantasy game, it would at least be fun for a while.
My initial plan was to play through it until I finally encountered something that would hand Zidane's ass back to him like Sephiroth had done to Cloud, then see if I felt like getting the guide book and trying again.
FFIX turned out to be the best FF game I'd played so far (it remains my favorite to this day). I had as much fun exploring the world as following the story, and one day, much to my surprise, I beat some big, nasty boss... and found myself watching the end sequence!
I had accomplished the impossible. I had finished a Final Fantasy game.*
So I decided it was time to show Sephiroth who was the boss. I still had my old save, and it turned out that I'd gone REALLY wild with the grind session - Cloud and crew had become insanely powerful.
Down the crater I went. An untold number of quadruple Knights-summonings later, Sephiroth was dealt with.
I sat back, enjoyed the ending, and it occurred to me that it had been TWO YEARS since I first bought the game!
(*For the record, though: Zidane DID get his ass handed to him. The final blow was dealt by Freya.)