Sam Langford by far should be harder, Fitzsimmons already fell to a proto Frazier in Jeffries, and Langford's short height would probably cause difficulties for Frazier's bobbing defense; he was used to being the shorter man.
Langford definitely. Although Fitzsimmons’ power is a wildcard, he makes an easier target for offensive-oriented Frazier. Langford’s versatility is dangerous.
I always found that Langford's combination of power, shortness and ridiculous reach would be interesting attributes for Frazier to overcome. It's not like Joe hasn't been tagged by punchers you could consider lesser than Langford like Bonavena and also Quarry to some extent. I still think Joe toughens it out and takes Sam out eventually though.
They'd both be steamrolled by JF. There's no evidence that Joe had trouble with smaller men, and there were no big dynamos like Frazier during Langford's and Fitz's prime years.
Hard to say, but both are very dangerous fights for Frazier, since Fitzsimmons and Langford were both very experienced and both had excellent right hands, which Frazier even at his best was very vulnerable to.