Dean and Crowley

Background
Dean and Crowley met during the early stages of the Apocalypse, after being told by Beck Rosen that Crowley was in ownership of the Colt. Much to the Winchester brothers' surprise, Crowley offers the gun to them without a fight. This small alliance gradually develops into a constant on-and-off thing, during which they are sometimes enemies and sometimes allies. Unlike Sam, Dean was more trusting of Crowley when they first met. Crowley also considers Dean a "bestie".

Dean and Crowley are never truly friends, but frequently become allies against much bigger foes. Unlike Sam, Dean was more willing to trust Crowley when the demon first offered to help them defeat Lucifer.

The two become enemies following the aversion of the Apocalypse, with Dean threatening to kill Crowley so he would break Bobby's deal. Dean's desire to kill remains even afterwards, though he is stopped by Sam. Crowley further angers Dean by kidnapping Lisa and Ben, so Dean stops hunting for him. Dean doesn't stop however, and continues trying to stop Crowley so he can't open the doorway to Purgatory.

Their alliance is restored when Dean and Sam require his help in defeating the Leviathans. The alliance breaks once Dean returns from Purgatory, now hell-bent on either killing or banishing Crowley, locking him up in Hell forever. Dean assists in capturing Crowley in the episode Sacrifice to enable Sam to turn Crowley into a human so the Hell Trials can be completed.

After keeping Crowley captive in the bunker for some time, Dean turns to him for help in freeing Sam from Gadreel's control in Road Trip. Dean reluctantly agrees to give Crowley his freedom in exchange and lets him possess Sam to communicate with him. Crowley's efforts allow Sam to expel Gadreel, but while Dean is grateful and allows Crowley to go free, he warns him that he'll kill the demon the next time they meet.

Despite this threat, Dean reluctantly allies himself with Crowley again to take down Abaddon by locating the First Blade. Dean and Crowley hunt the blade together, finally locating the demon Cain who is the creator of the blade. Cain eventually gives Dean the Mark of Cain so he can wield the Blade and helps him and Crowley escape after telling him where to look: the bottom of the ocean. However, while Dean got what he needed, he is furious with Crowley as he realized Crowley used him after seeing him let Dean fight off three demons alone when he could've helped. Crowley confirms this, saying he needed Dean's help to locate Cain and to get him to help as Cain would've never helped Crowley. Disgusted with what Crowley has done, especially as it resulted in the death of hunter Tara, Dean promises to kill him once they've dealt with Abaddon, but sends him after the Blade first as for the time being, he and Crowley still need each other.

During Crowley's search, Dean repeatedly calls him for updates and when they find the First Blade, Crowley releases him from where Cuthbert Sinclair has him bound, allowing him to save Sam. Dean is annoyed when Crowley takes the blade out of mistrust but protects him from a "hunter," not knowing it's really a demon testing him for Crowley. Eventually, Crowley helps Abaddon lure Dean into a trap to save his son, but he secretly warns Dean of the fact, using a code-word Dean had taught him. During the final confrontation with Abaddon, Crowley warns Dean of an attacking demon and seems slightly shocked by Dean's savagery in attacking Abaddon's dead body. Afterwards, as Crowley warned him, Dean lets him go rather than kill him while he's helpless but is angered when Crowley helps his son escape rather than return to his own time.

In the season 9 finale, Dean and Crowley get really close with Dean realizing his powers regarding the First Blade. Crowley also shows compassion and pity for Dean after the latter's death but gives back his blade, bringing Dean back as a demon. The two are together for a noticeable amount of time, with Crowley actively trying to become Dean's friend. Dean, even as a demon, disregards Crowley. To help cure Dean, Crowley goes to Castiel and feeds him another angel's grace to make this possible.

Crowley and Dean are back as enemies when Crowley decides to take the newly released Amara under his wing, and chooses to kill Dean to rid himself of the hunter. It is because of Amara that Crowley is forced to spare Dean's life. Crowley also briefly develops a dislike in Dean over Amara's favoritism in the hunter.

Dean seeks Crowley out again when he finally agrees to help Sam contact Lucifer. They acquire Rowena and the Book of the Damned and make a plan to go the Hell. Crowley later tells Dean that Sam went to Hell without Dean and is stuck with Lucifer. Dean and Crowley then help each other bind Rowena since she had been working with Lucifer all along, and force her to do their bidding.

It isn't until weeks after the unsuccessful attempt to re-seal the archangel that Dean and Crowley meet again, this time so they can use the Horn of Joshua to defeat both Amara and Lucifer. Crowley bails on the Winchesters when their plan starts to fail. They reunite when God offers to help fight the Darkness.