Originally the ones I'm gonna say were very popular at their time but nowadays they're just a cult classic (and rightfully so too). Mine were Golgo 13 and Lupin 3rd.
Golgo 13 is a great anime to watch because it doesn't require you to know any story whatsoever, and only 3 episodes have a tiny bit of lore involving his son and arch nemeses and such. He's an assassin and womaniser (which is shown in detail on the first episode!) who rarely talks and is almost featureless. His favourite weapon is the custom M16, which also serves as a sniper rifle as well as an assault rifle, which in my opinion is awesome!
Then there's Lupin 3rd, the world's greatest thief and in my opinion, one of the most charming men on the planet. He works alongside Daisuke Jegan, who can draw his sidearm within .25 seconds, and Guiomon (dunno how to spell that name, gwy-a-mon) the monk-samurai thingy. Then there's also Fujiko Mine, the love of Lupin's life, damsel in distress but also one of the most kickass women I've ever seen in the feature-length film, The Castle of Cagliostro. Lots of nudity, swearing and gunplay but still all very tasteful and you can tell it's from the 70s with its funky soundtrack and wacky animation! ^^