My unpopular opinion is basically *insert really popular game here* really isn't as good as everyone thinks it is.
Contenders would be Half Life 2 (oh dear god so much Hlaf Life 2), Portal (Or pretty much anything else by valve), GTA4, and probably one of my biggest bugbears, God of War.
Now I didn't play God of War on PS2, since I didn't have one, but I was recommended it when the God of War collection was released. And I honestly didn't get the appeal. I accepted that the graphics quality was going to be PS2 level, but the controls were shonky, the fixed camera was almost unbearable, the hit boxes were all over the freaking place, and the story... uh, hmmm...
What I disliked most about God of War 3 through (based upon the making ofs on the disc) was the sheer arrogance of Sony Santa Monica. 'Yeah, this game is the best looking one that is going to be released on PS3, in fact its going to be the best game ever released on the platform, period' Uh... No. Just know.
My guilty pleasure? Nudity in games. Yup, I admit it. I enjoy looking at digital boobs.
Don't judge me.