which among these mythics is good for a hammer?
It depends but I would say cosmic orb or earthquake. Maybe even arc.
Arc is good for builds (especially frozen) if you insta kill with it or close to it and it chain with things such as that frozen explosion. Earthquake, its more of a be close up thing, usually I see it being used by dagger blinkstrikers in pvp.
Cosmic orb, I think that is also a good choice because if you can attack fast enough, a good amount will produce where u aimed it. Since your build likely isnt spam too much, you can contril where u aim cosmic orb. Its a really good mythic for sure.
Nevertheless, test some of them and see how well you do. I would say cosmic orb imo but its up to you really 
Im gonna add to that, arc is good for must builds but isnt always the best possible MH mythic to use… Its great for its auto aim and can poison, freeze, paralyze etc to five enemys without you even needing to look at them, which is nice.
Earthquake is good if your pulling enemys together maybe through aftermath, the hammers primary attack or through taunt from totem… It has a decent aoe, basically the same as hammers special attack.
Cosmic orb is awesome for straight ahead, it may be slow but it lasts for a decent amount of time, it shoots out little sparks for what i believe is at 100% of weapon damage while the core deals 200% weapon damage. Targets behind you will be unaffected unless you pull them to infront of you, this is quite a good one for arena
But obviously it depends on your overall intention for the build? Straight-forward DPS your best with earthquake, survivability-arc is great for this and adds some extra targets which you never evene need to think about, cosmic orb- one of the best for arena, if you kill them before charging them you will benefit from this most although its low damage it can still create a little bitnof extra damage for you, so up to you 
Here’s a list of the damage that each one does:
Cyclone - 300% MH
Bombard - 250% MH
CosmicOrb - 300% MH
RockBlast - 200% MH
Earthquake - 300% MH
Arc - 350% MH
Apocalypse - 300% MH
Vanish - 100% OH
Nova - 250% OH
Enigma - 250% OH
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I always use CosmicOrb 