Suggestions for a online co-op game to play with a friend?

Don't Starve Together

I wanted to suggest Battleblock Theater and Path of Exile but you already have both.
You could take a look at:

• Rampage Knights. I had a lot of fun with that one both in singleplayer and co-op.
• Broforce
• Mercenary Kings. Fantastic game, even though I enjoyed playing it alone far more than in co-op.

Borderlands 2 is really fun in coop and offers a rather old school shooter experience (without cover system) and has a really good and crazy story with lots of weird and funny characters.

I was also gonna suggest Warframe since it's F2P so if you don't like it, not much is lost but apparently you've already tried that one. I personally didn't stick with it for long because it's rather grindey but 2 of my buddies enjoyed it quite a lot until they got stopped by a buggy story boss fight near the end of the story.

I don't know if the loading times are as bad on PC as they are on console but GTA V might also be fun with the new Doomsday Heist. Just keep in mind that if you're playing with just 1 friend you'll still have to deal with 2 randoms which in GTA Online can be a bit of a wild card.

For the Future you might also wanna keep an eye on A Way Out.

Edit: Since you already own Portal 2 have you tried coop mode yet ? If I remember correctly it has an extra coop campaign.

Rampage knights
Serous sam 3 bfe
and if you want to go on a totally different route you should give portal 1-2 a go

The games I enjoyed online with a buddy (that fit into your budget very well):
Trine games (2 and 3 can be played online)
Left 4 Dead (buy the second as it contains the first as well)
Resident Evil 5 (though we only just started it recently, it starts clunky but made me want to play more)

ccc-combo breaker:
Dead rising 3 could be fun, but I only played with random people, hence it is a black sheep in this list:D

Trine 1-3 have nice co-op, though some puzzles become quite easy with a friend. Watch some gameplay from youtube first though.

Borderlands 2
Trine 1+2
Magicka 1, 2
Helldivers (need somewhat more coordination than Magicka because of reloading time - it needs some strategy like not reloading the same time, covering the other. Feels like a pinch of military tactics in a Magicka-ish game. I really enjoyed playing it)
Forced - needs massive amounts of cooperation to get every goal and challenge.
Maybe Dying Light? People literally call it "the good Dead Island"

+1 To anyone saying Dying Light, but make sure to get the complete edition because the expansion is a blast.
Left 4 Dead 2 is a must
Serious Sam 3 BFE was so much fun to play and just to screw around with friends, Normally goes on Sale for $3
Battleblock Theatre and Castle Crashers are a blast to play with 2 people and is quite entertaining, but Castle Crashers supports up to 4
The Forest feels like an actual game finally and they've really improved on a lot of issues
Human: Fall Flat recently got Online Multiplayer and a friend and I blasted through most of it in 1 day because it was so enjoyable
Mount Your Friends is a fantastic game to play with a few friends
Saints Row 3 and 4, because Gamespy servers don't exist anymore. Otherwise, I'd recommend only 2.
Screencheat - A bit nauseating, but is quite enjoyable
Terraria - A fun minecrafty game that works well with online
Visceral Cleanup Detail - It seems like a boring game, but it gets interesting and frustrating especially if going for achievements or trying to 100% clean an area.
Vermintide is an amazing game if you can get at least 2 others to join you in an adventure. The DLC is a bit offputting though.
Zombie Army Trilogy is great if you can get 1 or 2 others to join you and just kill large amounts of zombies in the Sniper Elite fashion

There's many others, but these are the ones that I've enjoyed the most and have the best experiences in.

Division was fun.
So was The Crew.

If you like card games you can try out spades. It's a 2v2 card game which can be heaps of fun.

Dying Light / Killing Floor 2

Some great suggestions so far thanks guys I will consider a few of them. Time to ZzzZZZzzz now

I've made curator page where I list some co-op games, you might find something there that nobody has mentioned yet.
http://store.steampowered.com/curator/29958364/

You should check out Blood and Bacon, it's pretty cheap, and is actually a pretty good fps, and the multiplayer works quite well.

genital jousting

Aragami

Borderlands 2 of course, GOTY, don't buy without DLC

Left 4 Dead 2 is a must have. It's crazy fun and you need to work together a lot
Vermintide is like Left 4 Dead but with rats. I haven't played it yet but it looks fun
Borderlands 1 and 2. My bf and I recently played these both back to back and its super fun with a friend or more. Most people skip to the second one, but playing the first one added a lot more to the sequel, even though its a little dated. Overall very fun games.
Blood and Bacon is a cheap and silly fps where you work together to kill and survive angry pigs and earn power ups. The dev is constantly updating it and often plays with other players
Magicka is a fantastic co-op rpg but it has some downsides. It seems to crash more when playing online then local, and it has friendly fire so your spells will hit your friend if they are close. Other then that, its funny, addicting, and overall fun.
Human Fall Flat has just added online co-op. You and your friend can customize your character and go through the worlds together. It's a physics based puzzle game.
Saints Row 3 I think Saints Row 2 and 4 are also co-op, but I've only ever played 3. Think of it as Grand Theft Auto, but much more silly. You can play with a friend, and for the price it's a steal.

Sorry if you had some of these games already. I started to check your games but after a while my lazy side kicked in :P

I also found this website that has a huge list of online co-op. Good luck finding a game!

gang beasts,human fall flat.

Try Barony. It's extremely cheap too.

Be aware of lack of story, perma-death, rogue-lite elements and voxel graphics though.

• Risk Of Rain
• Blade Symphony
• Starbound
• Terraria
• Magicka 2
• Golf it

Don't know your favorite genres but these games are solely suggestions.

how about the division and space hulk:deathwing? btw elderscrolls online is realy nice to play with a friend i liked how it reminds me gothic games

Why don't you try this site.
http://www.co-optimus.com/games.php

Portal 2 is the best co-op experience I've ever had. If you haven't played it already, it's a must.

Have a look at The Wild Eight.