There is no one 'best' team you can choose - since, as mentioned previously, you don't know what you are up against - and not everyone has access to every type of Pokémon.
Instead, here is a general list of recommended Master League Pokémon to build a team from, with a wide range of sources that should suit all players, whether you've been collecting Legendaries since their debut, or have just started playing.
Remember, when forming a team, you are only allowed one of each in the Go Battle League, and ideally you'd want to build a team with different type strengths and defence.
Our Pokémon Go Master League Team recommendations in no particular order:
Giratina (Origin ideally, or Altered)
Type: Ghost / DragonGiratina Altered moves: Shadow Claw (Fast), Dragon Claw or Ancient Power (Charged)Giratina Origin moves: Shadow Claw (Fast), Shadow Ball or Ominous Wind (Charged)Giratina weaknesses: Dark, Dragon, Ghost, Ice, Fairy
Not only does Giratina rule the Ultra League, but it stands tall in the Master League as well.
Giratina is, put simply, incredibly powerful, leaning on its brute strength and ghost / dragon types to take down most other creatures in a straight match up. Though you'll want to go after the Altered forme if you have the option, Origin is still a welcome alternative.
Giratina is a Legendary and until it reappears in Raids or weekly Research, cannot be found easily without trading.
With the meta forming around Giratina, expect to come up against plenty of counters against it (which is most of the choices on this page) so though it's a popular choice for a reason, going against the meta by not using it also makes sense - so don't worry if you don't have one available.
Dialga
Type: Steel / DragonRecommended moves: Dragon Breath (Fast), Iron Head (Charged)Dialga weaknesses: Fighting, Ground
Dialga is another Legendary with the right moves, typing (steel is generally very useful in competitive play) and the strength to roll over the vast majority of Pokemon it comes up against.
You could argue Dialga is a better choice than Giratina. It's about as powerful, has fewer type weaknesses, and it being harder to find makes it a more predictable counter for the opposition.
That said, expect to see well-rounded teams with fighting or ground moves to still be a threat against it.
Togekiss
Type: Fairy / FlyingRecommended moves: Charm (fast), Ancient Power and Flamethrower (charged)Togekiss weaknesses: Poison, Steel, Rock, Ice, Electric
Togekiss is another Ultra League favourite which translates to a very strong Master League showing.
This is because Togekiss is an excellent Giratina counter - its fairy fast move Charge can take on a Giratina and fighting types such as Machamp with ease, while Ancient Power is a good counter to edge out another Togekiss.
Flamethrower, meanwhile, is helpful if you need to counter steel types - which it's weak against.
Togekiss is the best fairy type in the game, and also relatively easy to get your hands on - chances are you've hatched a few in Eggs over the years - and pretty cheap to level up and add a second charge move on provided you do it in its early evolution stages.
Dragonite
Type: Dragon / FlyingRecommended moves: Dragon Breath (Fast), Dragon Claw (Charged)Dragonite weaknesses: Ice, Dragon, Rock, Fairy
Having been subject to a Community Day and around since the start of Pokémon Go, Dragonite is a popular choice to add to your team.
It has one of the highest CP caps in the game, giving it brute strength few other Pokémon can match, and works as a very efficient Giratina counter with its dragon moves and typing.
Just watch out for Togekiss, a meta favourite, and other dragon types - including Dragonite itself.
Kyogre
Type: WaterRecommended moves: Waterfall (Fast), Surf (Charged)Kyogre weaknesses: Electric, Grass
Kyogre is another Legendary which stands out for its raw strength and bulk alone.
It's also, as far as Legendaries go, has been relatively common - so chances are you have one if you've been playing Pokémon long enough.
Though its water typing doesn't seem particularly useful in the current meta, outside powerful choices such as Dialga and Giratina, it should win out against the likes of Togekiss, Metagross, Snorlax and Mamoswine.
Mewtwo
Type: PsychicRecommended moves: Psycho Cut (Fast), Psystrike and Shadow Ball (event exclusive Charged)Mewtwo weaknesses: Bug, Dark, Ghost
Despite Mewtwo once standing as one of the most exclusive Legendaries in the game with its EX Raid debut, chances are you've gone out of your way to get a Mewtwo over the years, and unsurprisingly, it fares well in the Master League.
Ideally you'd have Psystrike and / or Shadow Ball (two event or legacy moves) to make Mewtwo the most effective it can be, but even without them, a powered up Mewtwo stacks as one of the most powerful attackers in the game.
Metagross
Type: Steel / PsychicRecommended moves: Bullet Punch (Fast), Meteor Mash (Community Day exclusive Charged)Metagross weaknesses: Fire, Ground, Ghost, Dark
With steel proving very effective in the Master league, having the best Pokemon of its type - Metagross - comes in handy.
Metagross will work well against Dialga, Togekiss, Dragonite and Mewtwo, but will struggle against fire types (Heatran), ground and fighting, as well as Giratina.
Though Flash Cannon is effective, to have Metagross at its best, you'll want Meteor Mash, its Community Day exclusive move.
Alternatively, the hard-to-get Melmetal is another great steel alternative, especially since it comes with fighting move Superpower and Thunderbolt, which is very quick at powering up, and useful for any Flying typings.
Pokémon Go in 2020 continues to add new features, such the arrival of the Go Battle League - which is currently highlighting the Master League - and soon, Clone Pokémon. We've also recently had the Buddy Adventure feature, which introduces a variety of perks - including finding Souvenirs - as well as a new trade evolution feature. 2019 saw the release of the first Gen 5 Pokémon, along with new regional exclusive Pokémon, Egg charts, and the introduction of the Unova Stone. Pokemon Go also sees regular events - including weekly Spotlight Hour and Mystery Bonus evenings, monthly Field Research rewards and Pokémon Go Community Day, plus a new Giovanni counters quest - the latest being The Shadowy Threat Grows.
Snorlax
Type: NormalRecommended moves: Lick (fast), Superpower or Earthquake or Outrage (Charged)Snorlax weaknesses: Fighting
As with other leagues, Snorlax is a great option because it's strong defensively; it's weak to very few other types, and has general bulk to stay the distance in a fight.
With Giratina also running rampant, Snorlax works as a good counter here with Outrage. But if you can, couple it with a second move - Superpower is great against another Snorlax, while Earthquake can handle steel types.
It is also, like Togekiss, one of the cheaper and easier to find Pokemon on this list simply because it's not Community Day move dependant or Legendary, and counts as a strong third choice if you're struggling to round out a team.
Mamoswine
Type: Ice / GroundRecommended moves: Powder Snow (Fast), Ancient Power (Community Day exclusive Charged), Avalanche (Charged)Mamoswine weaknesses: Fighting, Fire, Grass, Steel, Water
In a meta with Pokemon with several ice weaknesses (Giratina, Togekiss) and ground weaknesses (Dialga), Mamoswine's dual typing positions it as a very effective counter.
Mamoswine needs two charged moves to be at its most effective, ideally with Community Day exclusive move Ancient Power - though Stone Edge is also available.
Mamoswine's dual typing also has a disadvantage - more weaknesses. This means the steel types its strong against can damage it easily, as the mighty Kyogre, which you're likely to come up against.
Ultimately, it's a very good counter pick - especially when you factor in pre-evolution Swinub being a fairly common creature to make it a cost effective option.