Hypothetical arguments over who is "stronger" are a cornerstone of nerdism. Whether you've got bros arguing about which car would actually win in a drag race or UFC fans using MMA math to decide which fighter is going to come out on top in a bout, there's no shortage of people getting heated over who or what is more powerful in any given imaginary or impending scenario.
Is Supergirl stronger than Superman?
Superman has oft been decried as a "boring" superhero given the fact that he possesses a ridiculous number of powers, almost always does the right thing, and is often considered to have a somewhat "blank" personality.
It can be argued that the real dramatic tension of the Superman intellectual properties isn't whether or not the Man of Steel is going to come out of a conflict alive; he almost always will.
The real point of conflict is if Clark Kent will be able to save those around him, which could be why Superman movies always seem to fall flat. People are flocking to theaters to see the last son of Krypton stop bullets with his chest and fly around, yes, but the real heart of the narrative should be whether or not he can save the normal human slobs around him.
And DC Comics pretty much confirmed that Supergirl is better equipped to do that job than the most iconic hero of all time.
In Crime Syndicate #5, the Supergirl of Earth-3 takes on a bad version of Superman. While they're both Kryptonians and have the same DNA, Supergirl actually lived on the planet longer, meaning that she could have developed herself as a more skilled combatant under "normal" conditions and learned more of her people's ways than Kal-El ever could have.
In this comic, the characters go by the names Ultraman and Ultragirl, but their abilities and strength levels are presumably the same. So when Ultraman goes to punch Ultragirl, she easily overpowers him, stopping his blow with one hand and bringing him to his knees.
It's been chronicled in other DC comic series that the alternate-earth versions of superheroes and villains do pack the same power punches as their multi-dimensional counterparts.
So because Supergirl lived in the harsher planetary conditions of Krypton longer than Superman, she may have the physical upper hand. And there are also various storylines that depict Supergirl becoming a way more capable hero in the future than Superman ever became, so there's that.
Even though this suggests DC comics confirmed Supergirl is stronger than Superman, there are still folks who beg to differ.
One of the oldest nerd arguments about Superman's powers is that he's always holding back. In Crime Syndicate #5, it's unlikely that the evil Ultraman would ever hold back from destroying someone, but that hasn't stopped folks from employing that argument. (Check out the comments section of this Apocalypse clip where Superman fights a brainwashed-by-Darkseid Supergirl.)
There are some who argue that the only advantage Superman has over Supergirl is that he has more experience in using his powers situationally than Supergirl does.
Personally, I think that the whole "who is stronger than whom" debate when it comes to fictional characters is a bit pointless in the grand scheme of things. That's because we all know that Frank Castle as the Cosmic Ghost Rider is the most powerful comic book character ever. Duh.