The Power Stone has the power to annihilate all life on a planet in a matter of seconds, so it should be more than powerful enough to take down Doomsday, assuming his ability to absorb damage/energy isn't limitless.
Even though most uses of the Power Stone in fights was meh.
While that is true, we've only ever seen "Eson the Searcher"(The Celestial) use it to that extent. Ronan didn't do a tenth of that much damage with it. Maybe because he didn't know how, but we know for a fact he never showed enough energy projection/manipulation to overcome Doomsday. He's also weaker than him in h2h.
Wasn't the final act of the movie that they had to stop Ronan before he could destroy Xandar with the Power Gem? The narrative seemed to indicate he could do it.
Yeah, Ronan was going to use the Power Stone to wipe out Xandar, but then the whole Star-Lord's dance off shenanigans happen. So, in terms of overall power output, he should be able to. However, because we don't actually see him do it, we have no idea how fast he could do it and how much effort it would take (unless we assume it'd be similar to when Eson did it). Because, unless it's pretty much instant, there's every chance that Doomsday could just rush in and beat him to a pulp while he's attempting the attack. Because, in a straight fight, Ronan stands zero chance against Doomsday, and all Doomsday needs to do is hit him hard enough to drop his weapon once for it to be game over.
__________________ Never take life seriously. Nobody gets out alive anyway.
Yeah I think Doomsday wins more likely than not due to being much faster and more aggressive, along with physically powerful enough to dominate him. But if Ronan can use the Power Gem I think he one-shots Doomsday. The narrative seems to make it pretty clear Xandar was in serious danger of being destroyed.