We don't really know much about All Might in his prime other than he was 60x stronger than when he fought Noumu.
Judging All Might from when we see him in the series, Meruem outclasses him in speed and, arguably durability, but All Might clearly throws harder punches.
Meruem had a hard time getting through Netero's prayers, and Netero could throw a thousand of them a minute while All Might could only throw 300 blows in a minute in the show.
Judging from what he says about his prime, we could say that Prime All Might could throw 18,000 hits a minute or 300 a second which would make him much faster than Meruem.
I don't really like doing this much guessing when it comes to fights so I will conclude my thoughts with All Might from the show and then expand on possibilities for Prime All Might.
Meruem is faster than All Might. This is a given since All Might can only throw 300 blows a minute and Meruem was almost outclassing Netero who could hit 3 times as fast.
All Might is stronger than Meruem. All Might has far better strength feats than Meruem has ever displayed. Even without being in his Prime, All Might is throwing blows that change the weather, create shock waves, destroy city blocks, etc. Even Uvogin, who focused virtually entirely on his strength, couldn't pack the punches All Might throws around.
Now durability is tricky. I think we can all agree All Might is far more durable than Netero. He takes hits from massive power houses and comes back for more. However, Meruem essentially needed a Nuke in order to finally slow him down. Everything Netero threw at Meruem, over the entire fight, barely slowed Meruem down for a moment. He also barely felt any pain when it came to being hit by Netero's prayers. I think Meruem takes durability because he has better feat showings and All Might basically spits up blood randomly as he moves.
Conclusion: Meruem would likely defeat the All Might we saw in the show. He is smart in battle, composed, and fast enough/durable enough to give him some time to figure All Might out. That being said....... it would be a much harder fight for Meruem than his fight with Netero was. All Might won't be standing around waiting to be attacked and he will hit MUCH harder than Netero did. All Might will likely earn far more respect from Meruem given All Might is not only strong but he embodies Meruem's new ideals of using strength to protect the weak.
However, Meruem stands as champion against show All Might.
That being said.......... I think there is more than enough reason to make an argument that All Might in his prime could stand a solid chance of beating Pre-Rose Meruem in a fight. If he really is 60x stronger than he is in the show, which I point to as he said in his prime it would only have taken 5 blows to beat Noumu, then I would wager that the victory would go to Prime All Might