You don't get much more unsavory than a guy whose name literally means "Devours His Prey." As a Trandoshan, the deck was stacked against Bossk from an early age in the biologically deterministic world of Star Wars. His reptilian brothers and sisters were renowned for killing and enslaving Wookiees. Bossk wasn't much different which often put him at odds with Han Solo and his hairy pal, Chewbacca.
Despite repeatedly trashing Boba Fett, Bossk was pretty much defined by the many times the more highly respected bounty hunter kicked his scaly tukhus. It was an odd relationship, given that the two actually worked closely together during the Clone Wars. At times, Bossk was almost like an older brother to the prepubescent clone con.
Bossk went even further off-character when he helped Force sensitive and proto-rebel Ezra Bridger. On the animated series Star Wars: Rebels, the two work together to expose corruption in the Imperial ranks. Granted, even the Empire wanted this particular official brought to justice. The subsequent favor curried with the equally corrupt higher-ups could be why the Empire hired him years later to capture Han Solo.
Bossk's shaky loyalties and moral code continued for most of his career until he became one of the lucky few in his profession to retire alive. Not before he discovered that his supposedly dead rival, Boba, was still alive and on Tatooine, however. The two tangled in one last game of stubbornness. The Trandoshan finally won, and his renewed respect in the underworld meant he exited while on top.
While all of the hunters on this list have a history of working in teams, few of them have what you might call actual friends. The Gand Zuckuss and his protocol droid companion, 4-LOM, were different. The two became fast friends after being paired together by one Jabba Desilijic Tiure, also unimaginatively known as Jabba the Hutt. Together the pair worked first for the Hutts, then the Empire (hence their work for Darth Vader), and finally the Rebel Alliance. Sadly, their redeeming exploits would not be rewarded with peace in their golden years.
A member of the isolationist Gand race, Zuckuss started his career as a well-known and Force-sensitive tracker. When the Empire came to visit (and subsequently subjugate) his insectoid brethren, the tracker buzzed off to the stars to find new work. His connection to the Force, which he called "intuition," made him quite the catch for any scum or villain looking for a bounty hunter.
Before meeting Zuckuss, 4-LOM had much more humble beginnings. The protocol droid was just that -- an interpreter for the luxury star-liner Kuari Princess. Rather than dealing with diplomats and academics, however, he was a glorified text-to-speech program. He interpreted the ship's computer to its passengers. 4-LOM knew that being the futuristic equivalent of Siri wasn't all that fulfilling, even for a droid. He turned to stealing from his passengers out of boredom.
The pair's reputations brought them to the attention of Jabba. The Hutt quickly partnered them off and the bounty hunting dynamic duo was born.
This is around the time they fell out of favor with the Empire. After getting a snootful of poisonous oxygen, Zuckuss found himself in need of some new lungs. Unfortunately, the Galactic Empire wasn't big on universal healthcare. At least as far as illegally cloned organs were concerned. 4-LOM was only happy to help his best buddy. They took a desperate and dangerous job for the Rebel Alliance and incurred the Empire's ire.
With nothing to lose, they decided to continue working for the Rebels. That is, except for their brief and unexplained change of heart to capture Han Solo. But through the magic of a massive and unwieldy fictional universe they immediately became good guys again! In order to show how much the duo attempted, and failed, to rescue the target they had just tried to capture.
This put them at odds with Boba Fett, just like seemingly everyone else in the entire galaxy. Zuckuss had been restored at this point, so it was 4-LOM's turn to bite the bullet. Or a blaster, as it were. Fett did in his computer brain. Zuckuss was forced to wipe his buddy's memory to save his him. All that remained were the droid's senses of malice and greed. The two parted shortly thereafter. 4-LOM took to hunting solo, remembering none of his benevolence or ambitions. Zuckuss fell to schizophrenia and Force-induced paranoia (which are apparently diagnosable giant insects). And, with the original Star Wars expanded universe nixed, it's unlikely that the two will ever find reconciliation.