Minecraft.
My Kid is nearly 5 and is obsessed with it. We limit him to an hour a day, but he would play all day if he could.
He plays on easy, and creative mode, so there is no challenge, no danger, just a 3d construction kit.
It's been incredible to watch him go from just throwing random stuff all over the place to building his own villages and houses and castles and stuff.
I've even started making him put signs around and helping him, letter by letter, to write the words on them himself to squeeze even more education into the game that's already taught him so much.
We play some split screen adventure mode, and we share equipment and stuff leading to heart melting comments like "We make a great team don't we dad!"
We've really bonded over Minecraft, he's also played a lot of Lego games (loved them) and Little Big Planet (create again, he loved this before getting Minecraft) but nothing has really grabbed his enthusiasm like Minecraft has.