Meltan

Gyarados has the shortest zero to hero reign. It had Waterfall for a month before Kyogre first appeared.

I expect Melmetal to be fairly good due to it being hyped up by the Pokemon Company and Niantic. Also, Meltan and Melmetal are not only being advertised to sell Pokemon Let's Go, but it's also our first Gen 8 pokemon so it really has a lot of work to do. It'd be a bad move if they let their "mascot" for LGPE and Gen8 be a failure in Pokemon Go. First impressions do matter.

For how stupid busted Metagross and Meteor Mash are they'd need to give Melmetal a signature move completely pants-shittingly frightening to dethrone it. Like milkz said I expect it to be good, whether or not it'll best Metagross is another matter.

A premium product can't be compared with a free product. It effectively needs extreme amounts of investment or lets go catridges.

We have no idea of the stats but they do have to be very good to outclass Metagross if the moveset doesn't include Meteor Mash.

Regarding how many you can get, I believe it was announced that you can keep farming more Meltans by trading with a Let's Go and getting those boxes that act like Incense for Meltan. So maybe you can make more than one Melmetal because of that, but it's also a 400 candy evolution so might take quite a bit of effort.

If I understood correctly, the only ways to get Meltan is to trade it or to trade some pokémon with Let's Go. I don't plan on buying Let's Go just to catch a pokémon. I am and will remain a Go player only. And I'm not sure I want to spend the dust required for getting a pokémon unknown in your pokédex for that. So unless I missed something or the rules change, I don't care. I'll look somewhere else (and probably try to ask people to trade me a couple of MM Metagrosses!).