>>714516080 (OP)edf is imo the most fun coop game available so look forward to it.
As for where to start, its a bit complicated. Ideally I would say 4>5>6 , 4 has some really great maps and moments but can be hard to return to after playing 5 for all the improvements and gore system it added. 6, youre not gonna get the full appeal of the time loop story without playing 5 first, the catch 22 being that if you have played 5 its a lot of recycled shit and the whole thing feels like a dlc. Also they ruined air raider in 6.
5 is my personal favorite