Nine Parchments

Nine Parchments is what happens when someone decides friendly fire should be a selling point rather than an oversight. Four wizard apprentices hurling elemental spells at monsters and occasionally at each other, with enough perk and spell variety to make repeat runs feel different. It is lighter on RPG systems than anything in the top half of this list, which is worth knowing before you buy it expecting Diablo-level depth. What it does have is immediate, chaotic couch co-op that works especially well with people who do not normally play RPGs. I would use this to introduce someone to the genre before graduating them to something heavier.