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. 

Pick two from these:

Double Iron Bash isn't particularly strong, but it is a very spammy STAB option that lets Melmetal cleave through Fairy-type Pokemon. Superpower blasts through Steel-type Pokemon and has a great offensive profile overall, but its self debuff requires careful playing so Melmetal isn't taken advantage of. Thunderbolt can smash Water- and Flying-type Pokemon and can deal respectable damage to opposing Steel-type Pokemon, which may alleviate the need to self-debuff with Superpower in some matchups.

Outclassed options:

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. Rock Slide was once a staple move on Melmetal, but its nerf has relegated it to being a niche coverage option that can pressure some Fire- and Flying-type Pokemon