Diablo 2 and Lord of Destruction...
Nothing compares to the sheer amount of time I dedicated/wasted/invested (there's no "good" terms anymore, I think they all apply to some extent) with that game. It wouldn't be an exaggeration to say that if I compressed all the time I spent/wasted on it then it would probably end up being months (if not a year and plus) worth of non-stop sessions. I remember first buying D2 (vanilla) I believe during the summer of 2001 (not sure anymore) and "stopped" playing it (my first real "attempt" at quitting it) somewhere in 2005. I actually sold my copy (with LoD) at a local store at the time. Then the same year, maybe three or four months after I sold it I ended up buying it again (and brand new copy) and plunged into the addiction wagon (or call it obsession, maybe more of that than actual "passion") until - once more - I decided it was "finally time" to stop playing it, which was sometime in 2008.
My "journey" with D2 lasted approximately seven years of my life, and again no other games has come remotely close to that. Not only of the amount of time spent playing it to start with, but also to the "dedication" I had when playing it. Not only have I played it a crap load (of time), but when I did during the most "intensive" years (say between 2002 and 2005) I used to almost exclusively do that. It did negatively impact my social life. I managed to still go at work at the time... but I was so "obsessed" by it that I couldn't stop thinking about my next Baal runs and my newly-created characters (and their builds, etc) while at work. I came back home, barely took care of my real life priorities and jumped on my computer to be at it until the sun showed itself again the next morning. Being tired at work for lack of sleep due to a video game? Oh hell yes I know what that feels like believe me and it didn't last for only a week.
As for the highest number of hours put into it... in one sitting? That would be probably from late on a Friday after coming back from work, playing a good 14+ hours (keep in mind I usually had 8 to 10 hours of work on my shoulders on Fridays at the time when I came back home) non-stop (I.E. literally not standing up from my chair for that long not even to go take a drink) until finally I would have had to go to the bathroom, maybe take a drink, maybe blink my eyes a bit... then back at it until the early hours of the Sunday. I honestly don't even want to try to calculate how many hours I could spend on that thing I just have shudders when I think about it.
Yup... D2... heh, not exactly "proud" of having played "a game" [insert any name] for seven years or so. In retrospect I can only shake my head in shame when I think about it. But it's easy to say this now, because at the time I enjoyed every seconds of it. Put me back in that same context again and I wouldn't do any self-introspection to reason me out of it... I'd still be at it the same way. It is only now after I'm done with it that I realize. Damn you Blizzard North. I'm happy, actually, that D3 ended up being overall disappointing. I couldn't play it for more than maybe three months until I pretty much stopped playing it, good thing... yup... good thing.