Melmetal

Melmetal has the unique honors of being the first Pokemon from the main series generation 8 to be featured in Go, the first legendary/mythic to debut outside of the main series, the first Mythic pokemon to actually evolve, and the first legendary/mythic that can be placed as a defender as a gym alongside its pre-evolution, Meltan. Despite all of these honors, just how does Melmetal stack up in the Pokemon Go metagame? Not very well. With Melmetal's pure Steel-typing and lack of a Steel-type fast move, paired with Thunderbolt or the awful Flash Cannon, make it a poor option for raids. 

Double Iron Bash has finally been released, and it has excellent parameters.  However, Melmetal's average ATK stat and access to only Thunder Shock as a fast attack diminishes its potential as a Steel attacker. It can work as a budget option for those without Metagross, but the performance difference between the two Steel titans is rather overwhelming.

Best PvE Offensive Moveset

• Thunder Shock is Melmetal's only fast move. 
• Thunderbolt and Rock Slide pair well with Thunder Shock.
• Double Iron Bash* is an excellent move, but Melmetal lacks the Attack stat and Fast Attack synergy to make good use of it.
• Flash Cannon provides STAB damage but does poor damage. 
• Hyper Beam does little to help Melmetal.
• Superpower doesn't have much use here. 

Best PvE Defensive Moveset

• Thunderbolt is okay, but it leaves Melmetal easily walled by Ground Types in particular.
• Double Iron Bash* is a 3 bar move with a fairly early damage window and decent power, making it an excellent move on defense.
• Superpower is decent as an alternative move, but is easily dodged. 
• Rock Slide is very likely going to be resisted by common Fighting-type attackers. 
• Flash Cannon and Hyper Beam are one-bar moves that are too slow to access. 

Thunder Shock is the one and only fast move available to Melmetal. 

Rock Slide is Melmetal's primary move of choice. It's cheap, spammy, and has great offensive coverage. Double Iron Bash is an alternate option, providing STAB and even more spamminess at the expense of some coverage. Pair either move with Superpower for a big hit against other Steel-types.

Thunderbolt, Flash Cannon, and Hyper Beam are all outclassed by those moves, although Flash Cannon can be used to land a surprise, extremely heavy hit against Zacian. If you were to use Rock Slide/Thunderbolt, you should use Probopass instead. Flash Cannon and Hyper Beam just aren't that valuable.