Hand to hand only. Fight takes place in a boxing ring. Flash has his legs (the original ones) and no symbiote (as in depowered). Win by KO or submission. Both are wearing boxing trunks.
Flash should theoretically win, he bested Valkyrie in sparring and out H2H's any of his enemies in terms of skill. But...none of Flash's opponents (other than Valkyrie) really have proven hand to hand chops. With that being his only feat, Frank probably wins. My gut says Flash, but my brain says Frank 6-7/10.
@laflux: @dondave: Punisher wins due to pain resistance but your underestimating Flahs by ALOT. He's keeping up with Valkery an asgardian warrior, trained by SHEILD, professional boxer, years of being in the marines and training and experience as a fighter. He's had his neck bitten by Cletus Kassady and still fought on. Punisher wins but Flash is no joke just sayin lol