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.
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.