Lupin III

They share mainly stylistic choices, plot patterns, dialogues and tones. They also share the trope of a group of skilled but poor "outcasts" that live adventures in many different and distant places: they spend their time between solving dangerous troubles, finding themselves in unusual situations, trying to find money and getting something for their own needs or sake. Characters also share some personality traits, like the taciturn skilled buddy or the cynical girl. There are also many references in Cowboy Bebop that are obviously tributes and homages by Shinichiro Watanabe to Lupin III. 

If you dig Lupin, you'll probably dig Cowboy Bebop, and vice versa. Jazzy cool adventures with laid back badassery. 

Lupin III is essentially Cowboy Bebop's cartoonier grandpa, taking itself a lot less seriously and staying purely episodic (as opposed to the overarching plot that occasionally appeared in Bebop), but having a lot in common stylistically, with a similar episodic structure, and a main cast that's uncannily similar to one another. 

Cowboy Bebop seems to be spoofed off of Lupin the Third for a reason. Although they're still both dark anime, but they have two guys smoking, girls with attitude and guns at the same time, and overreacting kids. The main characters are also clever, too. 

Spike is Lupin, Faye is Fujiko, Jet is Jigen, and I guess Ed is in some way, shape, or form Ishikawa. At the very least, there's some obvious inspiration here. 

-They are both episodic -They both have a cast of shady characters (Lupin and Spike as the laid back protagonist, Fujiko Mine and Faye as the femme fatale and Jigen and Jet as the mature best friend) -They are both hand-drawn, have a 4:3 aspect ratio and were made in the 20th century -They are both classics in their own right and they both overflow with style -They both have excelent spin-off movies (like The Castle of Cagliostro, being the best for Lupin and Knockin' on Heaven's Door, being the only Bebop movie 

Lupin and Spike are two very similar characters in the lifestyle and how to deal with things, the characters that accompany the main are very similar too, both are episodic and very influential classics in world culture 

Cowboy Bebop's basically a serious take on the first Lupin III series (mixed with some Genesis Climber MOSPEADA nostalgic sci-fi elements). Spike's Lupin (and Pun), Jet's Jigen, Faye and Julia are both Fuyiko, and Vicius' Goemon right before joining Lupin's gang (they even had the same kind of gun vs sword duel, followed by a scene of our heavily bandaged hero lamenting himself). Borderline plagiarism. 

A crime drama series with a assemble cast of morally Grey characters with a Western inspired setting  

Both has that same character setting where the MC is a hustler by profession who deal in everything shady........however, when it comes to an antagonist setting, Lupid III was up against an overly zealot bumbling detective, while in GP the main character is in competition with his mentor where he hopes to beat him in his game one day.  

Both follow a gang of thieves with likable characters. Great Pretender is a more modern version with a storyline but Lupin III is a classic and just as fun despite it not having as much as a story. 

Characters have a similar dynamic. In the group of four their is a goofball with a serious side, a cool sharpshooter, a serious fellow, and confident redheaded lady. Both groups are also being "targeted" by an older gentleman who belongs to law enforcement. Also, both series follow a similar monster of the week type format. 

Cat's Eye and Lupin III both have their hereos/heroines steal an important artifact or a valuable item. They also use cunning ways to get their target artifact or item 

If you like Lupin III, then you will likely enjoy Mankatsu which has highly similar style, the goofiness and crime action that we know from Lupin. Not by chance, as it's also by Monkey Punch. 

It is no exagerration to say that Space Cobra is Lupin III, but in space.