I'm very love-hate with Rimworld. I have a lot of hours, but imo the base game is bad; it is however a great blank canvas for mods.

Can't recommend yet since it isn't out, but my hopes are high.

If you've ever played any of the Megaman Battle Networks then you need to try this.

Great physics-based, base-building strategy game. Long campaign, and PvP optional.

Even better than the original. This is a re-telling of the story, not a direct sequel.

One of my personal favorites, it is a must-try for anyone.

Amazing strategy game that I never hear get praise. 3 great campaigns and a wonderful co-op vs AI skirmish mode.

A classic survival co-op crafting game.

This one is a no-brainer. Good farming sim with co-op and tons of mods.

Relaxing hunting sim with full co-op features, if your friend has a DLC map they can host a game with it. A little pricey, but you can find sales and sometimes bundles on Humble Bundle.