It's one of these two.
The Elder Scrolls IV: Never Say Nerevarine Again
I am not wild about wide open fantasy role-playing universes. Maybe that's why I liked this one. I'm fairly confident you can't like Oblivion if you are a certain type of person. If you really liked Morrowind. If you are a fantasy or a stat nerd. And the backlash has been well underway for a couple of months now and it isn't really cool to like it anymore.
I am none of those people and I loved this. But I can't do much except reiterate the review I wrote.
There are funny quests, disturbing quests, draining quests, exciting quests, epic quests and none of them in isolation, all contributers to the game as a whole. An overheard conversation leads you to a character who leads you re-uniting him and his brother, which leads to a struggle to retake their family home in the middle of the woods which leads to someone finding you because he wants to find them. You pressure townspeople into testifying against a corrupt guard, he goes to jail, then, weeks later, he jumps you in the forest. Looking out from pretty much any point in the world and seeing the tower of the Imperial city. Getting gangbanged by two ogres, deciding to retreat, then, an hour later, you're still running, chased by the same ogres, two wolves, bandits, bears and a demon.
"Thank you for visiting our demonic cult, notice we have taken your weapons and equipment. Also please notice that we greatly outnumber you. Now, about ten of us will be waiting here while you prove your loyalty by sacrificing this prisoner. Here, you'll need this dagger. Go ahead, whenever you're ready."
All of it. It is the best game like this that I've ever played. I don't need to get tattoos or start my own library or get married or learn about a fictional agricultural system that will never be relevant or spend hours walking from the east coast to the west or play it with turn-based combat or learn Elvish or any of that noise. All there has to be compelling, exciting, meaningful game experience that doesn't make me feel like I wasted my time at the end of it, and, hey, look, there it is.
Half-Life 2: Episode One
Here's another favourite, despite a review which I think came out more negative than I actually meant. It's distilled Half-Life 2, so really, you already know whether you're going to like it. But for those who do, hey, that's great. It's better than Half-Life 2. It's faster-paced, there are no stop-start vehicle bits. The single-player-co-op works completely and Alyx is a great character. It's already got so much love in this thread I don't really know what to add.
Even though I'm ostensibly "unsure" about which is my GOTY, the fact that I wrote six times as much about one game than the other is a clue. Also I bolded one title and not the other. Episode One is of a more consistently higher quality than Oblivion (NOT hard to do when the game is five hours long and not 80,) but while Episode One got me psyched for Half-Life again, Oblivion totally fucking floored me. I thought it was going to be Morrowind but with better graphics and nominally better AI and that's not at all what it was. Maybe it's unfair that I'm giving it to the game which only caught me by surprise because I had no expectations for it. But, in the end, who actually cares.
Disappointment Total:
Dreamfall: The Longest Journey
Clearly this is not the worst game of 2006. It is, though, the one that frustrated me and disappointed me the most. And I am not even a fan of the original Longest Journey. I might have been able to take it if the story issues were enough to compensate for the gameplay issues (the gameplay issues being "the lack of any".) But both halves of the game are deeply flawed in such weird ways: writing which only ever alternates between needlessly blatant and needlessly cryptic, and gameplay which only ever alternates between badly implemented stealth/combat and walking from one cutscene to the next. I think for the next "spiritual successor" there should be some rethinking on why exactly this is a game. Oh, and then it would still need better writing.