It depends, if this is your first Mjolnir than whatever you like best should be the one. I had a pewter one for a while but I bought myself a pretty worthy solid bronze one that's actually a replica of a real one found in a Danish burial mound.
I prefer bronze because it's more authentic, but it tends to be more expensive. This one is on the low end price wise.
I also have a bronze replica of the Swedish Rus God Head pendants. It's hard to find, I picked it up in a museum in Normandy.
Whatever suits your fancy. I do like that last one a lot, it's not accurate at all but it looks totally badass. And that, in the end, is what counts.