Like most people in this thread. My first thought was the Klitschko brothers. They both had brilliants jabs.
It depends. Louis had a much better right hand than jab. Foreman was the same. Larry Holmes had a better jab than right hand, but together they were incredible. I wouldn't put Klitschko in this list because his resume is horribly thin. Someone like Louis would've sent David Haye into orbit.
Haye's slipperiness wouldn't have been easy for Joe Louis to land on to send into orbit and Joe Louis was kind of mechanical just like Wlad was.
That's why I don't think Ali and Holmes belong on this list. When you're talking about the combo and you have guys like Wlad with massive power in his straight right as well as a damn good jab, I don't see why you'd like someone like Ali or Holmes who were feather fisted compared Wlad.
For some reason Wlad came to mind. Since you specifically said the 1-2 combo. Just a mechanical piston controlling the fight.
Yes, his jab mostly worked to set up the right hand, and it's the most efficient and destructive jab-straight combo in heavyweight history, isn't it? That might not look stylish enough for the purists, I don't know. They tend to prefer better techniques, a more graceful way to land punches, and a jab that's equally powerful on its own. What defines "best", I don't know. * Wilder would have killed the Olympic gimmick Joshua with that combo.