Slightly off-topic, but do you guys think there have been lower scores across the board because the games are of lesser quality? Or because editors and reviewers are responding to criticism about score inflation? Or something else?

Bioshock infinite and Grand Theft Auto 5 is the correct answer.

I'd say Bioshock Infinite
If not, maybe Pikmin 3 or The Last of Us
If not, definitely GTA V

Looks like it could possibly be Bit Trip Runner 2, or does that game not count?

Maybe a response to the direction games are heading in, all becoming action shooter rpg hybrids, combined with franchise fatigue. Reviewers play all these games back to back, they would be the first to applaud originality and breaking the mold.

Bioshock infite and the last of us have a shot at 90+ if they can set themselves apart from the standard 'kill, gather, upgrade' grind.

Bioshock will be 84-88
Gears of War: 78-83
Last of Us: 88-92

I heard good things about Gears recently, but I don't think it'll reach 90. It's out of TLOU or GTA, maybe Bioshock. I'd be surprised if a next-gen game is first.

Uncharted 2 got 96, it's one of the highest rated games of all time. The team that made it is making TLOU. It's also got the zombie/post-apocalyptic and general Naughty Dog hype going for it. 

TLOU has high chances but after several reviews this year it will be tough, I think. GTA-V will make it, I think. It will most likely get the best metacritic score this year.

Grand theft auto V or the last of us