My all time favorite co-op games are: Helldivers 2, Vermintide 2 and Deep Rock Galactic. They are very much online games tough.

For offline local co-op my picks would be: the entire Trine series, Streets of Rage 4, Earth Defense Force 5 and Renegade Ops.

The previous installment had some good bones but they took their sweet time getting it to where it needed to be after release.

Hopefully the polish they added along the way is present at release in this one.

They really fixed the coop in the first (second) game recently. Even made the invasions opt-in. It’s now a really enjoyable 2-player coop experience.