Left 2 Die 

“In co-op mode, you and your pal will have to try and survive as many zombie waves as possible.” TouchArcade

Pocket Heroes

“Features like in-game chatting and bonus loot and xp for big parties make adventuring in Pocket Heroes as social and fun as possible.” GameZebo

Sonic The Hedgehog 4 Episode II

“Two players can take on the entire campaign via local Bluetooth multiplayer on iPhone, selecting who will play as Sonic and Tails at the outset.” Co-Optimus

Scotland Yard

“The boardgame is a co-op experience for most of the players, except for Mr X who must run on his own.  The new Scotland Yard iOS for iPad and iPhone aims to remedy this and can be played pure co-op if 5 players chose to go up against the Mr X AI.” Co-Optimus