The Sinister Five!

The Sinister Six they're not, but would you believe:

Comprised of the Rhino, Hydro-Man, our old buddy Speed Demon, Boomerang, and their leader, the Beetle (formerly of the Masters of Evil), the Sinister Syndicate banded together as hired guns for anyone who cared to bid on their services. It looks like their targets here are Silver Sable and Spider-Man, who were hoping to capture Jack O'Lantern but who got a lot more than they bargained for.

And though Spidey and Silver Sable gave a good accounting of themselves in the ensuing battle, the Syndicate eventually managed to get the better of them, by bringing almost the entire Coney Island roller coaster down on them:

Fortunately for this pair, the Sandman, who at the time had reformed (heh, "reformed," get it?) and was now enjoying life as a law-abiding citizen, decided to come to their aid and mix it up with the Syndicate--and doing pretty well, too, even with bonehead moves like this one:

This tactic also bombed when Quicksand tried it, so I really wasn't expecting any better of Sandman. But Spider-Man, injured earlier by the Rhino, rallied and desperately fought off the Syndicate along with the Sandman, until the villains decided to bolt.

The Syndicate would make one more go of it five years later, bringing the Shocker into their ranks and pursuing Spider-Man for revenge--but there was a power struggle between the Beetle and Boomerang, as well as further infighting which led to the group eventually splintering and going their separate ways.  There's yet to be any demand for the Syndicate's return, either by prospective employers or from readers banging on the doors of the Marvel offices--so it seems it's Doc Ock and his group whose sinister cred still sets the standard.