Which games do you think have the highest percentage/lowest percentage of user that actual finished the main story of the game versus the amount of actual people played the game in first place?

The games must have a single player focused quest/career mode to complete to count.

Least would probably be the Grand Theft Auto games. Depends what you count as "complete" really, there are lots of games that don't really have an ending but you can complete 100% of the content in the game. If those count, then I'd say Gran Turismo 5 is the least completed.

Any sandbox game will probably be higher, considering there is so much to do without completing the story. So I agree with the above^

GTA

Not finishing the game 100% as such, just to the point were you see a ending/credits screen by finishing the main story quest or career mode, etc.

skyrim - I played over 150 hours of the game and never finished the main quest

most is usually the more story focused games where people have a real impetus to continue forward. Heavy Rain had a completion rate far above the average iirc from one of Cage's presentations. If I had to pick one single game I guess I'd go with To the Moon. So short, the story is so gripping, and the challenges are minimal so I can't see much of a reason not to complete it.

Would be interesting to try and find some legitimate data on this stuff. Perhaps you could look for people with any trophy/achievement from a game and then use that as a total in comparison to those that earned the award for finishing the main story which almost every game has.

If you include getting all the badges in Forza 4 on top of getting all the achievements then it has to be included in the least completed games of all time. I'm not even sure if someone unlocked all the badges so far.

I thougt exactly the opposite. GTA series is one of the most complete because it's fun to do missions.

Getting all the trophies in GT5 is doable especially if you have a wheel.  Getting all the badges in Forza 4 is a lot more time consuming I'm not even sure if someone has done it yet.  Getting all the manufactures to level 25 which must be done to get all the badges probably alone would take over 1500 hours.

Mabye Skyrim, because it's so easy to get distracted