Probably Europa Universalis 4. I've played dozens of campaigns, and clocked in hundreds of hours (>850), and I have yet to make it more than 2/3 of the way through the campaign time. I usually have a specific win condition I'm interested in (e.g. an achievement, or cool scenario), and once I hit it, I don't bother continuing the campaign.

Other than that, I tend to bail on most RPGs around 50 hours in, so I've never finished any Elder Scrolls, Final Fantasy, etc despite starting numerous times. I feel like I bail when I get too powerful, need to grind because I'm not powerful enough, the story drags and I lose track of the main story line. I still keep starting them, but I almost never finish them.