Team Creation Basics
First off, there is no perfect team that's suited for every single eventuality.
With more than one thousand Pokemon there's a really ridiculous number of different teams that can be built, so it's pretty much impossible to create one single team that works in every single situation whether it's a competitive team or something more casual.
But that doesn't mean you can't be well-prepared!
Effort Values and Individual Values
EVs and IVs are a core concept in competitive team building, but they can also be used during more casual play too. EVs represent how well trained a particular stat is, sort of like body-building, while IVs are closer to innate potential.
EVs
The important thing about EVs is that Pokemon can have up to 252 EVs in a particular stat, with a total of 510 EVs which can be distributed however the player wants. Different Pokémon specialize into a particular stat, so a Special Attacker won't usually invest any EVs into its Attack stat.
IVs
Every Pokémon has an IV in each of its stats ranging from 0 (No Good) to 31 (Best) which affects how high its stats can go.
Trick Room teams exploit the fact that Trick Room makes it so that slower Pokemon move before faster Pokemon for the duration of the Trick Room effect.
This means that Trick Room teams can use ordinarily slow Pokemon to great effect, sweeping the usually-speedier opposing team before it can get a chance to attack.
So when making Trick Room teams it's incredibly important to pay attention to EVs and IVs, since a Speed IV of 0 with no EV investment is the best choice.
There are certain other scenarios where it can pay off to have 0 IVs in stats, such as for a Special Attacker who wants to reduce damage from Foul Play, for example.
But in general, having 31 in each of the useful IVs is preferred.
Core Pokemon
Typically you'd want to start by choosing your core Pokemon. These are the - usually two but sometimes three - Pokemon around which you'll base the rest of your team.
You can start with a specific Pokemon, or you can choose your favorite move, or even form a core based on Pokemon types - Grass/Water/Fire is a popular core! You can even choose your core team based on your favorites!
But once you've chosen your core Pokemon the real fun can start.
Next, you'll want to enhance your core Pokemon team by adding in Pokemon to round out their offensive or defensive capabilities.
For example, if your core Pokemon are weak to Flying types, maybe consider an Electric type Pokemon. It's always important to have a balanced Pokémon team.
Use Support and Utility Moves!
In most Pokemon games the story doesn't really need you to use support and utility moves like Dragon Dance or Calm Mind, but in competitive battles these can be make-or-break moves that a failure to use means a failure to win!
And it's not just about stat boosting moves either! Status moves like Confuse Ray or Hypnosis can be absolutely devastating when used in the right way!
But where moves like these can really shine is through the use of dedicated support Pokemon. These Pokemon aren't necessarily the strongest, most defensive, or the fastest Pokemon on any team, but they provide a whole lot of utility.
Take Sableye, a relatively weak Pokémon which, with the Trickster Ability, gets priority for its status moves. A real annoyance on any opposing team.
Sometimes the most powerful Pokemon can only win with a bit of support!
Support Pokemon will use moves like Baton Pass to pass along stat boosts to their team mates, or be ready with recovery moves like Life Dew, or defensive moves like Light Screen.
The best teams make great use of support Pokemon and utility moves, so it's always a good idea to have some solid support.
Think about type match-ups!
The best teams are usually ones that are well rounded and have good type coverage. It's impossible to build a team with every single type on it, but you can make good choices when it comes to moves by choosing Pokemon with a good move pool that offers decent coverage.
Steel type Pokemon like Corviknight are often a good choice because of how great a defensive type the Steel typing is.
In fact, the Steel/Flying type got a spot on this list of the best type combinations in Pokemon, while Corviknight itself got a spot on this list of the best Flying type Pokemon!
Fairy types are great too because of their powerful offensive capabilities and their total immunity to powerful Dragon type moves like Dragon Claw. Check out this list of the best Fairy type Pokemon for some inspiration!
It's extremely important to think about coverage, for example by giving moves like Shadow Ball or Dark Pulse for Pokemon which need protection from Psychic or Ghost type Pokemon! Many Pokemon get some really out there moves in their move pool, so some Pokemon have some really interesting options for type coverage.
Of course, it's always a good idea to consider same type attack bonus as well, so it pays off to use at least one move that's the same type as your Pokémon.
Be Aware of Your Team's Weaknesses!
It can be frustrating to build what you think is the best Pokemon team only for it to get completely wiped by an opposing Pokémon.
So it's very important to pay attention to your team's weaknesses. If you have multiple weaknesses to Ground type, address them! Find coverage or swap out a member for a different Pokemon, preferably with moves like U-turn or Volt Switch so you don't lose a turn. This is also a good way of dealing with stall teams!
It pays off to have a varied team with options for physical or special attackers. Some Pokemon will have high Special Defense stats so will need Pokemon with a high Attack stat to knock them down.
Other Pokemon will need a high Special Attack stat opponent to take them out, so it's always a good idea to have a mixture of Pokemon on your team - including defensive Pokémon for a really balanced team.
Example Core Team
A good choice of core team is one themed around creating a particular weather effect, in this case Rain, a good starting point for weather teams.
Pelipper
Pelipper is a really good Pokémon to use for setting weather effects such as Rain. This is because Pelipper gets the Drizzle Ability which creates a Rain weather condition when it enters battle. Rain powers up Water type moves and powers down Fire type moves. Pelipper can automatically create Rain without wasting a move!
Pelipper gets access to some great utility moves like Tailwind and Wide Guard. Perfect for weather teams!
Palafin
Palafin excels on a Rain team because it's a Water type Pokémon and a physical attacker with access to great Water type moves like Jet Punch and Wave Crash. Palafin can also use Haze, which resets stat changes of all Pokémon on the field.
These two Pokémon pair well on a Rain Team, which makes them a good example of a Core Team.
Main series PVP
It can be really intimidating to getting into competitive battling, but it doesn't have to be! By following the basic tips outlined earlier it can be easy to dip your feet in.
The most popular competitive battle format is double battles, which play out a little differently to single battle format battles.
In double battles the entire team is based around the doubles format. This means that a good competitive team will have a strong core, make use of support Pokemon and moves, and have one Pokémon chosen to balance the other's weaknesses.
Consider the metagame!
The best competitive Pokémon teams consider how other people are playing the game and make use of that.
Things like which Abilities are being used - for example, does a Pokemon use its Hidden Ability often in battle, such as Cinderace with its Libero Hidden Ability?
Or even things like which Pokemon have the Levitate Ability and which Pokemon are used for setting up weather conditions with moves like Rain Dance or by using their Abilities.
Weather teams are one thing, but Life Orb strategies, Stealth Rock, and especially status conditions are something to really consider.
The competitive scene is ever changing, but it's often enough just to consider things like priority moves and which Pokemon are often used to do which thing. And certain Pokémon will appear again and again on a whole range of different teams, so it's always important to consider this in your team building.