Which local co-op game should I buy?

Salt and Sanctuary (shared/split screen) and Crimsonland.

Towerfall Ascension and Broforce are outstanding, and one of my friends keep mentioning Hidden in Plain Sight as well :)
About divinity / helldivers - divinity is a massive, serious RPG, helldivers is more like magicka, in the more classic "clusterfuck" coop way - both of them seems to be fun in their own way, just decide for which one you're looking for

Can't say much about Divinity but I played Helldivers for a few weeks with 2 buddies (after it was free for PSN Members) and have to say I wasn't particularly impressed.