I know Handbook evidence don't count (it's stated there too) but I recall it mentioned on panel in an old Defenders issue that she was the strongest female in Asgard or something.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
^ I'd be happy to see that scan of a character statement. Even moreso, I'd be happy to see any scan of an actual feat that would show Valkyrie possesses the strength to effortlessly tearing apart a steel door like Sif has.
I don't remember what Defenders issue that was in. Her handbook entry backed up the claim though (but I know handbooks don't count unless on panel evidence is produced).
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
Now, I'm sure Valkyrie is at least as strong as Sif is. She is an Asgardian, and even more than that, she's a Valkyrior. I just don't see how, without any specific feats in mind, that anybody can be so confident that she's physically stronger than Sif.
Again, I'm welcome to seeing actual feats proving so.
According to Marvel Wikia (which seems to be pretty legit):
Valkyrie:
Superhuman Strength: Brunnhilde, like all Asgardians, possesses superhuman strength. She is significantly stronger than any other Valkyrie, and most other Asgardian men and women for that matter. She can lift up to 45 tons whereas the average Asgardian male and female can lift 30 and 25 tons respectively.
Lady Sif:
Superhuman Strength: Like all Asgardians, Sif is superhumanly strong and possesses greater physical strength than the average Asgardian female. At her peak, Sif is able to lift about 30 tons.
If this is so, Valkyrie is 1 1/2 times stronger than Sif.
Im pretty sure those strength levels were copies from Marvel themselves, and to me they do make sense and are believeable from what I know of these two characters. Believe them or not, I was just trying to help.
^ I appreciate the effort, but those wikis are completely editable by anonymous users. In fact, I can edit Sif's entry right this very moment if I wanted to with a description saying, "Sif has exhibited strength enough to tear steel doors apart, beyond anything so far shown in a comic by other Asgardian women, such as Valkyrie." You can edit it too.
Fan descriptions are subjective. When discussing characters, I prefer to focus on what we see in an actual comic. As a reminder, I'm amenable to seeing a character state on-panel that Valkyrie is 1.5x stronger than other Asgardian women. It's not a feat per se, but at least it's a character statement within a comic we can work with.
Yes, I understand that, though from memory (as I had all of the Marvel Handbooks) those stats looked like they came from Marvel, and the site itself looks quite legit.