For me it has to be Whitaker/Robinson. Whitaker's slick defense coupled with Robinson's offense just makes for a fighter that can't be beat. Such a fighter would not lack anything, since both guys also brought superb intanginables, stamina and chin to the table. Your pick?
Willie Pep and Mike Tyson. Pep's boxing abilities combined with Mike's power and killer instinct. Having said that I do find this hard to envisage.
Since they both first and foremost was formidable offensive fighters I don't think they complement each other very well.
SRR is considered the best fighter puond for pound EVER. You can mix him with Butterbean and still have a nearly unbeatable fighter. His defense was superb until he started getting up in age. Joe Louis has patience, an underrated defense, and some of the most powerful, shortest punches ever thrown.
SRR:s defense was good, but I wouldn't call it superb. It was not like Whitaker's or even Leonard's IMO. His offense is probably unrivaled, though.
Look at some of SRR's early bouts. He was nearly impossible to hit. Little subtle movements made opponents miss punches by inches where he would still be in a position to counter. Even as he got older he still showed flashes of defensive genious. Whitaker, a defensive wizard, would make fighters miss, and miss badly, but never put himself in a position where he could immediately counter from that opportunity.
I'm not saying they are the two best fighters ever, but I think you would form the best fighter ever if you combined the abilities of Roy Jones Jr and James Toney to create a 168lbs fighter. Jones Jr 1994-1996 at 168 was, in my opinion, the closest there has ever been to an invincible boxer. H2H, he was incredible at that time. If you combined all his own skills and gifts with the defensive qualities and chin of James Toney, surely you would have some sort of superhuman fighting machine! Imagine Jones (who had a good defence himself but which was based on mobility and athleticism) with the ability, whenever it was needed, to slip, roll and block punches brilliantly when up close, to avoid ever getting hit flush, and (if he ever did get hit flush) to be able to absorb massive shots to the chin. Who could beat that guy? I genuinely think no-one in history.
Ding Ding Ding, we have a winner Or RJJ-Mayweather, SRR-Mayweather, Ali with Lennox's power (I choose Lennox because his punching style suits Ali more)
I don't think Mayweather's chin has proven to be as good as Toney's though, so that's why I went with Toney rather than Floyd to combine with RJJ. Both would work though. It's good to see some fighters of this generation being rated so highly in a discussion like this rather than just the inevitable 'Langford and Greb' or 'Louis and Pep' answers. :good