Half-Life for me, I can see why people would pick MGS, even if I never liked those games.
I like Half-Life because of the atmosphere, everything just feels so... real?
That's not the right word but it's like everything that happens in Half-Life can happen in this world. I'm not saying it's realistic (it certainly isn't), but something like that.
The characters are believable, and I keep wondering what they experienced before Gordon returned (HL2, that is)
to that we can add a solid game play piece, there's a certain "flow" to it.
I think it has to do with the fact that jumping increase your speed (along with other things, ofc), in stead of decreasing it (like in most games).
There's just something extremely satisfying about jumping over a table and pumping a combine full with lead, and then turning around to do the same thing to his friend.
It's hard to explain.
I don't agree with people who say that HL2 is generic, it looks like the standard FPS on the outside. But when you take a deeper look it's something beautiful.
HL2 is what convinced me that video games can be art.
Please don't flame me for sounding like a fanboy, I like other games too.