No. 4: Iron Fist & Luke Cage

Credit: Marvel Comics
Luke Cage and Danny Rand would not be characters that on would expect to be paired up together. Cage was a criminal put in jail and then experimented on, giving him his impenetrable skin. Rand was the son of the CEO of Rand Industries until his parents were killed in a horrible accident. After inheriting his parent's money, he traveled to K'un-Lun, a fabled city in another dimension and began a rigorous training regimen only to be rewarded with a fight-to-the-death against Shou-Lao the Undying (a dragon), in which Danny ended up pulling out the dragons heart to gain the title and power of the Iron Fist.
Because of Danny's unwillingness to use any of the money his parent's had left him, he set off onto the streets to help the ones he could, which was how he ran into Luke Cage. Going by the moniker Power Man, Cage was a Hero-for-Hire; a hero that would do the dirty work heroes like the Avengers wouldn't do. He did it for a living. Soon, they teamed up and both became Heroes-for-Hire. During Luke Cage's solo comic book run the book didn't do well in the stores, that is until Marvel decided to put these to heroes together into one issue, focusing on the many things they did for people who could hire them. Which is kind of poetic, because they saved each other, not only in the Marvel Universe but in our Universe as well showing just how strong their bromance was.