Castlevania: Symphony of the Night.
I stumbled across a used PSX version of this back when I was in first year university or grade 12 and picked it up for $20 or so. The guy at the checkstand was telling me how awesome it is, and how I really lucked out. I played it quite a bit, but probably only got a 70% or so completion score, and finished the game. Good stuff, but I never played through it again and eventually sold it or gave it away.
A few weeks ago I remembered hearing about the second castle and how the full game actually consists of a 200%+ completion rating. So, I decided to download it and give it another go. I forgot how awesome it is. This time I totally took my time and really explored every aspect of the castle. I missed a lot the first time, and I've been having a blast with it. I just entered the second castle, and I'm still going strong whereas I'd usually lose interest by this point.
I strongly recommend anyone with a 360 to buy this when it hits the marketplace in a few months, and for those of you with a psp; there is a torrent of a psp version out there, so if you don't find it morally reprehensible you may want to try that out.
We really need more games like this.