Local-Coop Favorites?

Rocket League / Enter the Gungeon / Ultimate Chicken Horse / Sonc & All Stars Racing / Spelunky / Orcs Must Die! (both) / Cuphead / Cook, Serve, Delicious! (just the first / ...

Trine, Portal 2, Stick Fight

nidhogg
They have released a sequel as well, but I haven't played that yet

Fireboy and Watergirl:
https://www.fireboynwatergirl.com/

Eon Altar, it's a real shame, they didn't finish the story.

havent played many, last one i'd played was Monster Prom

ibb and obb is quite creative and it gets progressively harder.
brothers a tale of two sons is pretty straight foward. enjoy once.
nuclear throne is awesome and endless... but very hardcore.
audiosurf is very fun and challenging.
risk of rain for some very good action and challenge.
victor vran if you want some interesting long gameplay.
shank for full awesomeness.

oh I must add that all of them I won here as giveaways or I was gifted by someone that I consider a true friend.
I want to thank you all (again) for every single title. I just want to register that, even if they don't read this. :)

Besides from those that were mentioned:

Heroes of Might & Magic III (Or any of them, 3 is just the best.)
Speedrunners
Crimsonland
LEGO Star Wars (Or any of them.)

Epic Loon
Crawl
Secret of Mana

First place: Overcooked 2 - its just awesome...
S.place: Overcooked xD
T.place: Fossil Hunters - really recommended
...Castle Crashers
... Brothers - A Tale of two Sons
... Lovers in a Dangerous Spacetime
... Degrees of Seperation

I know it was already mentioned a few times, but if you haven't checked out Lara Croft and The Guardian of Light yet, do yourself a favor.
There are not many local co-op games which absolutely require teamwork to advance and this game is one of the best designed games when it comes to working together imo.

And if you just wanna have some rogue-lite fun, Rampage Knights was a huge surprise for me.
Really good game.

If you are looking for something new, BOSSGARD looks promising. We only tried it for 1 hour though so I can't really recommend it.

Heroes 3. Oldie, but awesome.

Human Fall Flat is really fun to play with a friend. Great to do that first too, because then replaying solo is more challenging to do without help.

Killing Floor 2 is one of my favorites. I also love to play similar games like Strange Brigade, Zombie army trilogy, World war z.
Local coops with my son: Lego games, Castle crashers, BattleBlock Theater, Trine, humas fall flat....

I barely played local coops, lack of time (or lack of friends) to come over.
But Hammewatch and Towerfall: Ascension are bloody fantastic.