Best Sandbox games this gen: The Saboteur, Infamous 2
Worst Sandbox games this gen: Assassin's Creed 2, Assassin's Creed 3
Most overrated Sandbox games this gen: Assassin's Creed 2, Assassin's Creed 3
Most underrated Sandbox games this gen: Yakuza 4, Test Drive Unlimited 2
There have been a bunch of open world games released this generation and honestly, very few have actually been bad. The Assassin's Creed games, the entire series though, are garbage, and are extremely overrated. I've only played, and finished, Assassin's Creed 2 on the Ezio Trilogy and Assassin's Creed 3. While the second game was better than Assassin's Creed 3 overall, combat was worse and both games just sucked.
I have a lot of favorites in the open world/sandbox genre and want to show my appreciation for the genre by listing other games I enjoyed that didn't make it into the above categories. Batman Arkham Asylum/City, Dragon's Dogma, Yakuza 3, Just Cause 2, Oblivion, Grand Theft Auto 4, Sleeping Dogs, Red Dead Redemption, Test Drive Unlimited, Infamous, and Saints Row the Third.
I own but have yet to play Prototype 2, so hopefully that's another good game in my most popular style of games this gen. Never played the first.