Both were close to defeat plenty in their careers, Rocky was behind against Walcott and almost had the Charles rematch stopped due to his cut. Jeffries was close to losing against Sharkey (fight 2), Corbett (fight 1), and Fitz (fight 2) but managed to turn the tide. Who impresses you more?
Rocky.. he bulked up to 185lbs.. hed barely be 180 as a young man if he hadn't trained to be a HW boxer.. Marciano would probably be a SMW if he was fighting today. He's not much bigger than Arthur Abraham.
Rockys comeback vs Moore shoud be talked about as well and was also 1 punch. Faint left hook, chopping right started Moore's downfall. He was as much a clubber as he was a 1 shot artist.
Neither really. I will go with Marciano because he made the comebacks and whatever you think of his resume he went 49-0. With Jeffries theres a real possibility he lost Armstrong and Sharkey 2. We're really taking peoples words for it who didn't show their work and who were overly biased. In the sources about the Armstrong fight one could infer the authors thought Jeffries lost and might have faked his injury but are refusing to write the words.
I've no problem with picking Marciano, but he didn't bulk up to 185lbs. He was way heavier than that before he became a professional boxer. His training was heavy on cardio and his diet very light. He ate steak, swallowed the juices and spat out the meat. He had an extreme calorie deficit diet and training regime, designed to make him as light as he possibly could be, whilst in fighting shape.
Marciano was losing pretty badly to Moore who seemed to have him figured out but Rocky made some slight adjustments to his punching angles while never taking his foot off the gas and came back. Impressive stuff. Though I am not sure he could have done that to slightly younger Moore.