I don't pick Foster,I would pick Moore or Spinks though.Foster's physique generally could not cope with heavyweight punches.
Carnera did not ko Impelletierre who out boxed him early on ,the Imp was rescued by the referee.With all his size and weight advantages Carnera never duplicated that feat in a straight fight. I discount the Schaaf fight. I spoke to Gains at the RAH,he was happy to talk about his career but,beyond a smile,he would not be drawn on Primo's abilities.In his autobiography Gains said Carnera did not punch his weight. Louis said Carnera had nothing beyond a fair jab which he pushed. There is a story that to prevent Carnera having "wet dreams",his management put a rubber band on his *****,asked about it Angelo Dundee said ,"they coulda put 10 rubber bands on him ,they still wouldn't have made him into a fighter." Gains going to the floor was not counted as a knockdown he looks to have been caught square and bundled over and is getting up as soon as he goes down. Carnera Bubble Burst By Gains (youtube.com)
And significantly Carnera couldn't do much to him beyond stamping on his foot and breaking a toe despite having over an 80lbs weight advantage.
Did Bob Foster even register a single win at HW? He seems to be one of those guys that had no ability at all to climb weightclasses despite being the elite of the elite at his proper weight. Maybe it's just the guys he fought at HW but asking him to beat a true giant when he actually lost to someone of the similar size, I think Carnera stops the thin man brutally.
Yes, that I'm aware of, that he skirted the line technically above 175 a number of times but those IMO were still really LHW fights. I'm talking about the real HWs (Zora, Ernie, Frazier, Ali), not these fights that were 179, 180 etc.....I don't believe he had a win over a 200 lb guy (If I'm mistaken by all means I have no problem being corrected). I'll be honest with you, I've never considered the guys that were 180ish as HWs (including Marciano). I even say I consider Rocky probably the greatest CW of all time even though he was a "HW". I know the size differences were what they were then, but I would be considered a HW of yesteryear at 185 currently and I'm really the frame of a MW/SMW.
I'd go with Primo. People insist a lot of Primo's fights were fixed. But Tommy Loughran was a hall of fame light heavyweight, his fight with Carnera wasn't fixed, Loughran tried his best to win the heavyweight title, but Primo was just too big and strong. I don't think Bob would do any better. Bob wasn't a dancer, either. With Primo laying on him and muscling him around, Bob probably gets fatigued and stopped.
A smart tactic on his part. Unless you're Floyd Patterson, Max Schmeling, Joe Louis or a few others, odds are you use dirty tricks. Primo used a lot of them. Pulling Max Baer down with him when he was getting decked repeatedly was shrewd time buying. Both had excellent backhands. His head was in the game when Sharkey decked him with that monster hook in their first go around. He quickly got up, then took a knee to obtain the benefit of a full count, whereupon Jack lost his head as he commonly did, delaying and extending the count even further, buying Carnera even more time to recover. For all intents and purposes, Foreman really beat Qawi when he dropped Dwight with an excruciatingly illegal kidney punch. He lost that round with the resulting penalty, but that shot had to have been in Qawi's mind when he later threw up his hands and conceded in disgust. Tony Galento sparred with elbow pads. He actually got under the skin of Louis, really making the Bomber mad. Nobody else ever got off the deck with Joe to return the favor. Who was the cleanest fighting HW Champion? That would be a good question.
Get this...In 29 career bouts with 11 wins, Eddie Vick never produced a stoppage win. Even crazier, his only recorded knockdown I've ever been able to find was against Bob Foster! In their first bout, he even made Bob go the distance. BF's best HW stoppage win? I've viewed Liston-Besmanoff where Willi stood up under a great deal of punishment without going down. Bob knocked him out in three for the quickest exit of Besmanoff's career up to that time. After Besmanoff, a case might be made for Chuck Leslie 2X. Leslie had some good wins, draws with good opposition and competitive decision losses to household names. After BF II, he handed Ted Gullick his first career defeat and took Ken Norton the distance.