Six iPhone and six iPad games with co-op mode appear in the top 100 paid apps on the App Store right now:
iPad Top Paid Co-Op Apps – 12/26/2012
#5. Minecraft – Pocket Edition $6.99 
#19. GarageBand $4.99
#27. Call of Duty: Black Ops Zombies $2.99
#80. Call of Duty: Zombies HD $1.99
#84. Draw Something $2.99
#94. Sky Gamblers: Storm Raiders $4.99
iPhone Top Paid Co-Op Apps – 12/26/2012
#2. Minecraft – Pocket Edition $6.99
#8. Call of Duty: Black Ops Zombies $2.99
#31. Call of Duty: Zombies $1.99
#52. Zombieville USA 2 $.99
#58. GarageBand $4.99
#74. Draw Something $2.99