Any Good Co-op Online Multiplayer Games ?

Assassin's Creed Unity has some great coop, but it is singleplayer centric (meaning there are about 10 coop missions and the rest is singleplayer focused). The game itself is mediocre but the coop is good fun and really encourages working together. If you mean to find random people to play with though, depending on the time of day you could have some issues.

Destiny, Overwatch, Division, Wildlands, Arma 3, Killing Floor 2

Killing Floor is PvE

Killing Floor 1 or Killing Floor 2

helldivers, enough said

Insurgency, Rising Storm 2: Vietnam, Killing Floor 2, Mount & Blade Napoleonic Wars(if you join a regiment). The games I have listed are the ones people frequently use mic or chat. Also, I have not played but Squad. I have seen comments where squad leaders kick out people who don't use mic.

Orion:Prelude is fun co-op, and you can have a crapton of people playing at the same time.

Guardians of Orion is fun too, but you're limited to 4 people at a time unless you're doing one of the PvP game modes.

Left 4 dead 2, Trine 2, and Lara Croft and the Guardian of Light all have online cooperation elements

Magic duels has coop team play, Lost Castle & Rampage Knght are fun, stellaris & Plague evolved.
Minecraft and any HackNSlash like diablo3 grimDawn or Path of exile.

Payday 2, Dying light.

Killing Floor 2 is a 6 player coop pve , best of the kind

Borderlands 1, 2 and The Pre-Sequel. Dying Light.

Titan quest, Grim dawn, Victor vran

Some of the ones I have played and enjoyed to various degrees:

Lara Croft and the Guardian of Light.
Torchlight 2.
Ghost Recon Wildlands.
Portal 2
Sanctum 2
Warhammer : End times Vermentide
Left 4 Dead 2
Spiral Knights
Borderlands 2