Thanks for all the replies so far.
Flash potentially could rip Superman’s brain out. But why would he? He did it to Amazo because he’s an inorganic being. I have a hard time reconciling Flash pulling Superman’s brain or heart out of his body in a dust-up. Killing a living being isn’t exactly his thing. His best attack would be to BFR Supes into the Speed Force and leave it at that.
You make it sound as if Flash can kill Superman in one move at any time.
In my opinion, Superman's bones would be almost on a par with Adamantium so I don't really see Flash easily phasing through them. Of course the argument I suppose would be he can do it to (Robo) Amazo who is presumably made of some super-tough metal anyway.
Flash seems to pull out the “big tricks” against bricks he would have a hard time hurting physically (pre 52, he broke his hand just punching Grodd because he wasn’t moving fast enough; it’s a couple of issues before he fights Zoom for the 1st time).
I'm actually amazed the fight between the two hasn't been properly done (yet).
Flash's 'tricks' seem to suggest the writers are not entirely comfortable answering the tougher questions of what might happen in such a fight where one hits the other.
I mean if Superman (in anger) hits the Flash I presume that's the Flash dead? So they can't legitimately interact beyond Flash pulling some sort of trick.
I think it’s 50/50; Superman is fast enough to “keep up” and apply pressure, causing Flash to make a mistake and get tagged roughly half of the time.
That's how I'd imagine it would 'play out'.
The brief Justice League movie 'tussle' saw Flash dodge 6 blows but get downed by a shoulder 'shrug'. Note that Flash wasn't trying to attack Superman here so he didn't need to stay close, presumably if he did he'd have got tagged quicker.