DUnno about the Sinister thing. Personally I think living forever would suck....and the pasty complexion...probably works wonders for the ladies . . .
But anywho, if we are sticking with the spiderman stuff I would go for Spider-carnage. I always liked Ben better, and like many of you, I love the symbiotes, so it works for me.
If we aren't sticking to spider-foes, I'd go with Onslaught all the way. I mean, he's Magneto (undoubtedly the most bad-ass jew ever to grace comics) combined with the mental powers of Prof X. He could wrap you up in a fence and make you think you were a little girl, playing with a Barbie. Awesome.
And as for Mysterio....I think my sig pretty much wraps up my view on that guy.