Assume both fights are legit. Long periods of minimum fighting activities by both champs pre-fight. Note: Tunney fight scheduled for only 10 rounds.
I would have to say Ali over Liston I. Liston was prety close to the peak of his powers theoretcaly, while Dempsey was coming of a long layoff. Tunny Dempsey I was only an upset for people who did not know how far Dempsey had declined.
I think we would have to been around to realize how big an upset these fights were to the public. I don't have any idea about the Tunney fight. I remember the Clay Liston fight and it was a huge upset. Liston at 7-1 odds gives you an idea.
Dempsey went to hollywood and I'm pretty sure to insiders it wouldn't have been a huge upset. Liston was seen as the next Joe Louis tipped to reign forever but he got spanked by someone who was nearly outboxed by jones and nearly knocked out by cooper.
Ali win over Liston was bigger upset. Liston had a reputation similar to what Tyson had in his peak in 1980's. Many were afraid of him.