Favorite Couch Party Games?

Broforce
Duck Game
Gang Beasts
ibb & obb
Keep Talking and Nobody Explodes
Monaco
Move or Die
Nidhogg
ROCKETSROCKETSROCKETS
Skullgirls
Worms (any of them)

BADLAND: Game of the Year Edition
BattleBlock Theater®
Final Exam (2 Players only, if i remember rightly)
Nine Parchments
Not Dying Today
Shank (2P only)
Shank 2 (2P only)
STARWHAL (Up to 4 players)

No people to play with :(
But the few times we had, Towerfall Ascension was awesome, good mix of skill and chaos.
Hammerwatch / Heroes of HW can be fun as well, though it's not really the jump in-out partygames.
I see some awesome suggestions in the topic, I won't repeat them ;)

Eon Altar

Bloody Trapland
Crawl
Use Your Words
Ultimate Chicken Horse
8-bit Fiesta

Don't Starve Together (survival silliness)
Evolution (digital card/board game, Few games with that name)
Armello
Eon Altar
Carcassonne
Stone Age (https://steamcommunity.com/sharedfiles/filedetails/?id=389777819)

Actually, I find any digital board game is pretty fun to play with others because the games last just long enough to have a few matches.

Eon Altar is quite something. Very different from usual party games and being able to use phones as controllers makes it a breathe for more than two players
Final Exam is a fun beat 'em up but you are limited by controllers and keyboard in couch co-op

Battleblock theater , Move or die , Ultimate Chicken Horse , Speedrunners

Competitive:
Crawl [4 players] Highly Recommended!
Tooth and Tail [4]
Lethal League / Lethal League Blaze [4]
Rivals of Aether [4]
Disc Jam [4 in 2 teams]
Pyre [2]
Roof Rage [4]
SpeedRunners [4]
Rocket League [4 in 2 teams]
Screencheat [4]
Towerfall Ascension [4]
Nova Nukers! [4]
Invisigun Heroes [4]
Brawlhalla "FREE!" [4]
Fantasy Strike [2]

Casual:
Ultimate Chicken Horse [4]
Tricky Towers [4]
Blazerush [4]
BOSSGARD [6]
Grip: Combat Racing [4]
Move or Die [4]
Hidden in Plain Sight [4]

Co-op:
Lovers in a Dangerous Spacetime [4]
Helldivers [4]
Monaco [4]
Castle Crashers [4]

Epic Loon
Seriously, it's awesome.
Even though nobody knows it.
Good luck not killing each other. :)

Ones that I've really liked and will always return to:

• Frog Climbers this is such a great game
• Marooners franticly switching mini-games, it gets better once you unlock some stuff, though I wish there were more options
• Ultimate Chicken Horse adjustable rules and handicap scoring make it work for almost every group
• Think of the Children it's co-op insanity, and kind of hard, and very funny
• Human Fall Flat only 2 players, but fun to watch, and easy to hand off control
• Magicka or Magicka 2 funlarious in co-op and in versus
• Megabyte Punch it's like playing simultaneous, split-screen MegaMan and you can replace all your body parts for new abilities

Ones that are fun for a break between other stuff, but probably not more than half hour at a time.

• Clusterpuck 99 we find this one incredibly hard
• Penarium It's hilarious, but only 2 players and gets repetitive after a bit
• Neighborhorde it's a horde shooter, but kind of a meh one, unique weapons are fun though
• Circuit Breakers I just didn't click with the shooting mechanics
• Insanely Twisted Shadow Planet multiplayer is difficult if you haven't learned to play already

Longer time investment co-op games:

• Lost Castle I think this could be great if you can get into it
• Sacred Citadel fun side-scroller with progression and a bit of loot, you can do a level at a time and come back later

I still have tons more that I've acquired and not tried yet.