Sam and Crowley

Background
Sam Winchester and the demon Crowley are never friends, but repeatedly become allies throughout the series. Their first mutual adversary was Lucifer. Due to his previous experience with Ruby, Sam did not trust Crowley at all and tried to kill him many times. He did not develop much trust in Crowley unlike Dean when the Winchesters first met him. Crowley affectionately refers to Sam as "Moose".

Sam later unknowingly works for Crowley, collecting creatures alongside the Campbells who served Crowley. When Sam does find out, Crowley holds Sam's soul as ransom to force Sam to work.

Eventually, Sam works with Dean, Meg and Castiel in an attempt to kill the demon. When it is revealed that Crowley is still alive, Sam and Dean being to hunt for Crowley, also to stop him from opening Purgatory.

In season 7, Sam reluctantly allies with Crowley again to fight against the Leviathans, notably when they need his blood for the weapon that can kill Leviathans. Despite this, Sam still doesn't trust Crowley. After Dean kills Dick Roman and disappears with Castiel, Crowley appears and taunts Sam about being "well and truly on your own" and kidnaps Kevin Tran, refusing to tell him where Dean and Castiel are.

Shortly after Dean's return from Purgatory, Sam works with him to try and seal up Hell forever, so they could be rid of the demons, including Crowley. As part of the trials necessary to complete this task, Sam attempts to turn Crowley human in the episode "Sacrifice", even saving him from Abaddon, but stops at the last moment at Dean's insistence.

In season 9, Sam reluctantly works with Crowley to stop Abaddon. After Dean kills Abaddon, Sam lets Crowley go free but refuses to help him remove a devil's trap bullet, saying that letting him live should be enough for him. He gets angry when Dean works with Crowley to hunt Metatron and sends him away. In season 10, Sam yells at Crowley when he finally gets him on the phone and learns that he is sending demons after Dean. Crowley lets Sam track Dean down to force him to get moving and helps Sam capture Dean in order to stop him as Dean as a demon is completely out of control. Sam hands him the First Blade in exchange.

Season 11
When Crowley takes the young Amara under his wing, Sam joins Dean in infiltrating Crowley's palace in order to kill Amara. Sam defeats the demons standing guard while Dean goes into Amara's room. Dean later confesses to failing to complete the mission, much to Sam's chagrin.

Sam experiences mysterious visions which beckon him to seek out Lucifer. After Dean relents, the brothers seek out Crowley and Rowena's help. The plan was initially to wait for Dean to come back after checking out a crime scene, but Sam decides to go ahead without him and travels to Limbo, where the trio meet Lucifer.

Unfortunately, Rowena had been working for Lucifer all along and Sam ends up trapped inside a cage with his former torturer. Crowley thus calls Dean and informs him of Sam's predicament. With Dean's help, Crowley manages to force Rowena to cast a spell to send Lucifer back to the Cage, using a witchcatcher. Once Sam is safe, Crowley orders him and his brother to get out of Hell.

Sam and Crowley do not hear from each other for a couple of weeks. A demon informs Sam that Crowley has become Lucifer's bitch-boy, and Crowley himself, upon escaping Lucifer's clutches, tells the brothers of some of the things Lucifer did to him. Both Sam and Dean express joy at the Crowley's suffering.

With Crowley and Rowena's help, the brothers set up a trap for Lucifer with the intent of expelling him from Castiel. Their combined efforts accomplish nothing, aside from Crowley getting momentarily trapped inside Castiel until Sam expels him using an incantation.

Weeks pass by again before Sam and Crowley see each other again. God reveals himself to the brothers and devises a plan to use angels, witches and demons as part of their attack against Amara. The main members gather at a warehouse, though Sam and Crowley do not converse.

They remain on the same side following the attack's failure. While Crowley and the rest feel hopeless, Sam encourages them to think up another plan, and they end up using a Soul Bomb as their last resort. Dean is selected to carry the bomb to Amara, while Sam and Crowley and the others wait for the results inside an abandoned bar.

When the sun returns to full glow, Sam and Crowley along with everyone else deems the mission complete, and Dean to be dead. The hunter and demon therefore, go their separate ways.

Trivia

 * According to Jensen, the actors who portray Crowley and Sam (Mark Sheppard and Jared Padalecki, respectively) got along with each other from the moment they met.