We are kind of on Halo right now.
I ran through all the Halos back in the day, but intend to get the Master Chief collection for the xbox one and do the main story line again. Better system. Better TV. 
If you do Borderlands 3 be aware that the main complaint is that the text on much of the stuff is so small you can't read it without getting close to the TV. I thought it was just me until I looked on the 'net and found it's a consistent complaint.
I don't co-op but single player is fun. The game is long enough that it will last me with the small amount of time I have to play.
Oh, that tiny text is a problem sometimes. I have that issue when we do Minecraft with more than 2 people on local multiplayer. Minecraft is by far and away the video game we do the most in our household. It's just impossible to exhaust that game. We basically only do multiplayer games in our house, except for Forza. One of our other favorite games is Lord of the Rings: War in the North, but I couldn't recommend that to be played as a single player, because it was designed as a 3 player co-op, and it would be impossible to play that game with 2 computer-controlled characters.
I like strategy and shooters and so my thinking and mental coordination/reflexes are tested. + I just like blowing things up (on screen)!
Halo is the first game that my sister and I have played that requires a lot of strategy, and we are really enjoying the challenge, and the teamwork that comes with playing it together. And of course, the shooting up random guys.