My opinion: Venom had a few things going for him. First, he was drawn by Todd McFarlane and then Erik Larsen for his first 20 odd appearances. Even if they're not your cup of tea (I have my criticisms, but compared to the average artist of the day on mainstream superhero comics they were unique and interesting) they certainly grabbed attention. Secondly, he was a dark version of the hero which can be interesting if done right. A bigger version of Spider-Man with a monster's face. Of course, that looked ridiculous on Demogoblin, but hey. Third, he was a scary bag of powers including invisibility, limited shape-shifting, more physical strength than Spider-Man, and scariest of all he didn't trip Peter's "spidey-sense". Fourth, he knew who Spider-Man was, which wasn't an open secret amongst his villains at the time. Fifth, when he backed off from wanting to kill Spider-Man/Peter he became an anti-hero, and it was the era of anti-heroes. He had the whole violent/"kewl" look thing going on, like the Punisher or Ghost Rider or (as mentioned) Lobo.
Joker, smart/unpredictable/capable of doing anything. There isn't much he could do at this point that would shock you, where the Riddler or Penguin or even Bane you could see lines they wouldn't cross. Second, his movie appearances were by far the most interesting of the Batman villains (IMO, and apparently others, outside of Leto's). I could do without about a thousand "Joker" motivational memes on Facebook lately, but I get the character's appeal.
Lobo I think was meant to be a parody of the Wolverine/Venom type characters, but was still embraced by those who saw it that way and those who loved the Venoms/Ghost Riders/Wolverines/etc.
Just my 2 cents, but think Venom and Lobo were products of their era and the Joker is adaptable and unpredictable enough to be a good villain in any era.