Who is more powerful in the Arrowverse: Superman or Supergirl? The answer is far from obvious, as Kal-El of Krypton has far more practical experience as a fighter but Kara Zor-El has shown signs of possessing more raw strength than the Man of Steel.
The question of which of the Kryptonian cousins is more powerful has been a subject of some debate in the world of comics, with the answers varying based on which version of the DC Universe one considers. Supergirl is generally considered the more powerful due to her living on Krypton into her teens and being used to the stronger gravity of her homeworld; a factor often used to explain the super-strength of Kryptonians on Earth. Another factor is that Supergirl spent a longer period of time in deep space in cryogenic suspension, adsorbing the stellar radiation of many yellow suns and building up a larger energy reserve than Kal-El did on Earth. However, neither of these are a factor in the Arrowverse, where Kara Zor-El was trapped in the Phantom Zone for several decades (where there are no stars) before coming to Earth.
Given that humans and Kryptonians in the Arrowverse are close enough genetically as species to interbreed (a fact known thanks to Lois Lane having two teenage sons with Superman in the upcoming Superman and Lois series), it can be presumed that they are physically similar in most other respects, ignoring the Kryptonian ability to act as living solar batteries. As such, Superman likely possesses more physical strength than Supergirl, given that men generally possess more muscle-mass relative to height and weight than women. On the other hand, human women have a higher tolerance for pain then men, so Supergirl might be able to best Superman based on sheer grit and endurance.
Thankfully, the question was given a definitive answer in Supergirl's season 2 finale, "Nevertheless She Persisted." The episode opened with Supergirl being forced to fight Superman, who had fallen under the control of the Daxamite queen Rhea after being exposed to a piece of Silver Kryptonite. The precise effects of Silver Kryptonite were not defined, either making Superman paranoid or susceptible to mind control. In either case, Superman hallucinated that Supergirl was his worst fear; General Zod returned to Earth, seeking revenge.
In terms of power, the two heroes seemed evenly matched, with the beams of their heat vision proving to be of equal intensity and capable of blocking one another. However, Supergirl ultimately won the battle, finally knocking Superman unconscious after the two traded punches for an extended period, though she passed out from exhaustion afterward. Later, when he was back in control of himself, Superman confirmed that he was completely convinced that Supergirl was his worst enemy and that he had not been holding back. Despite having a decade's more experience in using his powers and fighting evil, the Arrowverse version Superman failed to beat Supergirl in a straight-up fight, confirming that the Girl of Steel is the more powerful of the two.