No one has a counter for Super Speed--and yet somehow they manage. I don't see Flash as particularly decisive here.
Assuming that Superman is in more of his "first amongst equals" mode as a JLA member, and isn't having one of his Hulk-out moments, then Superman, Wonder Woman, and MM are roughly equal to Surfer, BRB, and Gladiator.
So really, it's the undercard that is key: Kyle, Flash, and Captain Atom vs. Ronan, Nova Prime, and Quasar.
Hmmm....seems close. How uber was Nova Prime again?
Flash has higher highs, no doubt. But Flash is not, IMO, typically positioned as a heavy hitter in combat formidability--though he is highly variable and his good showings aren't just top tier, but tip top tier if you know what I mean.
But on the whole, Flash is typically a high-mid-tier to low-top-tier threat. And Ronan is in than range, IMO.
Realistically, sure. But then so is just about anyone.
In the comics, power set matchups tend to matter less than where each character is positioned in the hierarchy in terms of overall formidability. Top tiers tend to beat mid-tiers, regardless of whether their power sets should allow that. Writers just find a way. Hulk will hit the speedy, ranged flyer with that sewer lid or thunder clap eventually!
Exceptions do occur, of course. And more often in the modern age, but by and large the old conventions ring true.
But if this is a high-end Flash depiction, like versus the Anti-Monitor or whatnot, then Ronan can't match that--even on a good day. But I might give Ronan odds when both are having their typical day.
I personally don't aim for some kind of definitive asnwer when there isn't one in the comics. I don't use peak or not peak, I just try to describe the broad contours of the matchup. Sometimes it's pretty simple and a 7/10 or whatnot will do. Sometimes numbers won't do it, and all you can do is sketch the rough outlines.
That's my approach. Others differ. If it gets me kicked off the board, then I guess this wasn't the right place for me.