I mentioned it before, but I've always been a fan of the Sinister Syndicate (see reason #47). Beetle, Boomerang, Hydro Man, Rhino, and Speed Demon (and later, Shocker). I think they're regarded in general as a team of second-stringers, kind of losers, a very poor man's Sinister Six. And while it's true they aren't as evil as the Sinister Six, I still think there's a lot to like about them.
For one thing, I think the Sinister Syndicate does have at least one frontline Spidey villain, the Rhino. You scoff, but that's because in today's Marvel he's a punching bag for whichever creative team wants use him that month. If it's not Deadpool (when he's three inches tall), it's Frank Castle, or who knows who else. But Rhino is at least as much of an A-lister as an old dude in a bird suit, or a guy with a fishbowl (for the record, I like Mysterio, but that fishbowl is silly looking). Plus Hydro Man is arguably more powerful than everyone in the Sinister Six, except maybe Electro or Ock (On the basis of Ock's smarts. The tentacles wouldn't do much good against a water dude).
Beyond that, there's the differing motivations. The Sinister Six formed because the bad guys were tired of getting pantsed by Spidey. If they had actually succeeding in eliminating him, that probably would have been the end of their partnership, they go their separate ways and they each would have been caught by the Fantastic Four or the Avengers about a week later. By the time we get the "Return of the Sinister Six" (Amazing Spider-Man #334-339), and "The Revenge of the Sinister Six" (Spider-Man #18-23), they had set their sites a little higher (world domination), but everyone on the team was just jumping to Ock's tune. He suckers them in the first story, then laughs off their attempts to get him for it, then the second time uses his Adamantium tentacles to whomp the Hulk, and scare the other five into working for him (except Sandman, who was turned to glass). At that point, it's less a "team", more "Doctor Octopus' gang of super-powered helpers". For the record, I do remember that there was another Sinister Six during the Mackie/Byrne years, one run by Electro, and hired to kill Doc Ock. But that group also had Venom in it, so I'm choosing to call that out of continuity.
For the Sinister Syndicate, it was about moola. Working together, they could pull jobs with a greater rate of success, because five villains have a better chance of escaping the cops or Spidey or Daredevil, than one bad guy. They weren't going high-level enough to attract teams like the Avengers (who would have trounced them), so they were going to actually have a chance to make some cash. Their first time out they got hired by Jack O'Lantern to eliminate Silver Sable (who was trying to collect the bounty on Jack). That Spidey was there as well was just a happy coincidence for some guys with history wirh the web-slinger, not their primary motivation, which makes it a nice change of pace (probably because I still really like it when the villains are focused on more than just revenge on the hero).
Plus, the team didn't exactly work well. Not that the Sinister Six were all that chummy, each demanding to take their own shot at Spidey the first time, and by the later incarnations they were together mostly out of fear of Octavius. But with the Syndicate, guys got into it with each other. There was politicking. Boomerang was frequently challenging Beetle for the role of leader, and each guy would court the other members for their side (in Deadly Foes of Spider-Man #1, Beetle makes certain to side with Rhino in an argument over whether to kill Spidey, because he recognizes the value in having the Rhino's power and loyalty on his side). Speed Demon was the typically loud-mouthed jerk a lot of speedsters seem to be (because everyone moves so slow to them?). Because the uniting purpose was only money, the bonds weren't that strong, so you had the potential for more intrigue, backstabbing, and unpredictability, compared to a team that eventually was being run by one guy.
So in a way, it was a little gratifying to see half of the Sinister Syndicate in the Avengers: The Initiative #3 book a few weeks ago, up to their usually thieving ways. I'm not sure they should have gotten captured by a bunch of losers in Iron Spider armor, but I guess the point of that was to demonstrate that SHIELD will have their own Spidey's, if the original won't play ball, so what ya gonna do? Maybe the whole team will get back together as part of that MODOK's 11 maxi-series.