I find, that playing a game that you had absolutely no expectations to be good and having it surprise you, is better than playing a big name game. Playing a game like MGS4, I expect the world handed to me on a silver platter and often I say, "Oh well this is good, but not really what I expected going into the game."
Case in point? On the same exact day my brother purchased Mass Effect 2, my friend loaned my Valkyria Chronicles. Now, on Valkyria Chronicles I saw the words "Sega" and instantly my expectations where grounded, as I had played so much as a decent Sega game since Sonic Adventure 2. Now, by the time I got to CH3, I was enthralled and Mass Effect 2 has collected dust ever since. My friend may just have to tear this game from my cold, dead, fingers.
Favorite/latest unexpected diamond you've uncovered?