I just finished what is possibly the best game of all time, and I don't take that phrase lightly.
Before playing MGS4, I proudly proclaimed that Chrono Trigger was the best game of all time. But after finishing MGS4 I now realise that it's a close second.
I'm not gonna say exactly why it is (as I don't want to give out any spoilers or anything), but trust me... it is.

Feel free to talk about it too if you've experienced it. But please do try to be kind to others who havn't played it yet (and plan on it) and if your going to reveal anything important, please leave a spoilers marker so people know what parts to skip.