Elite 2 Froniter, because there is no definitive "ending" to the game.
I think the best you can do as far as completing it is to get Elite Ranking, as well as get the highest ranking in each of the factions to get the Thargoid Mission.
Then on-top of that there are a number of missions that are spread across the colonised portion of the galaxy, some of which are difficult to find or require special permissions to enter the system which military institutes don't provide only black market operators do.
Plus once you do them all there is no "game over" credits screen, but instead you can still trade, get new ships, do more missions, etc. it's a simple case of you personally achieving them. For one of the original "sandbox" style games, it frankly is the only one with truely no ending. Even GTA games you eventually have nothing to do because you've done everything, where-as Elite keeps randomly generating new missions for you to take on.
I suppose it does lack any in-depth storyline, but as far as it goes the Single Player is technically infinite. Just a case of how long it takes to get bored of, nowadays I think the graphics would put most gamers off it; but back in 1993 when it was released I wouldn't see friends for weeks at a time after school when they said they just wanted to try getting an Imperial Courier for some contract jobs. Plus it was possible once you have a fuel scoop on a hydrogen-based engine to skim suns to never run out of fuel. Let you explore the whole galaxy provided you were a damn skilled pilot not to die doing so. I got about 100,000 lightyears round before my Amiga 500+ died on me.
As far as heavily storybased single players go, I think one of the longest you'll find is probably the original Fallout. The new Fallout 3 has quite a similar storyline, but in terms of depth it's like Fallout 3 is the abridged 3D cliffnotes.
Most of the Final Fantasy games generally are long but as a rule tend to be only about 30hours of gameplay if you just go from story area to story area. It's the overworld that expands the game as well as small side missions and the want to complete everything.
Longest RPG I've played was the original Grandia, my brother told me it has 80hours of basic storyline in it; I think it took me around 140hours in all to complete it. I'm not so good at RPGs.
There are so many games people could choose for this but the problem is games like say Fallout 3, personally I have a game that has ~200 hours of gameplay on it; but the first play through where I did nothing but the main storyline was closer to 14hours.
I think if you're going to compare lengths of games it should be for the adverage gamer who will play just the basic storyline. GTA4 is a good one because even going through just the basic storyline missions it is extremely difficult to keep under 30hours of gameplay hense why there is an achievement for it.