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.
I thougt exactly the opposite. GTA series is one of the most complete because it's fun to do missions.
All 300,000 of them?
I love San Andreas to bits, but even I had to force myself through half of the missions. Some of them were brutal too.