I'll second what others pointed out earlier : a balance team needs flexibility. And that might be the only goal one should aim to fulfill while building a balanced team.
Basically, you want most of your pokemons to be able to always act both offensively AND defensively.
The synergy of the defensive core should be almost flawless, this gives them staying power. But shouldn't give them "setup bait" power, at no cost.
This can be done in a wide variety of forms :
- If a defensive pokemon can U-Turn or VoltSwitch, then it has an offensive potential in the sense that it grabs momentum.
- Perish Song is great, because it prevents setup AND force switches. Good paired with fast offensive pokemons or U-Turners with type synergy with the Perish Song user, because this minicore has the upper hand at least for 2 turns.
- If it carries TW, it has an offensive potential since that means your slower offensive pokemons will be able to act first later on.
- An offensive pokemon with a lot of usefull immunities/resistances (gengar), is also very defensive if used correctly.
A pokemon like Mandibuzz, as presented earlier in the topic, is a perfect illustration of an awesome balance team member for a large amount of reasons :
- It has awesome physical bulk AND very good special bulk.
So it can be used as a backup special spounge to cover lacks of type synergy within your core.
- It has staying power in Roost
- It can use Taunt, Defog or Whirlwind to remove hazards or prevent setups
- And the most important thing : it has Foul Play, so it can spounge physical hits AND kill physical attackers
It is both extremely efficient in offense and defense.
You can actually consider it to be a physical sweeper somehow, since when it comes it's on physical sweepers and it nails most of them. That is, when you call it for its job, it is perfectly capable of killing.
If the opponent switches on a defensive pokemon, Mandibuzz can Taunt/Roost/Defog/Toxic to give you the advantage. If it switches on a special attacker, it has enough bulk to endure a hit, it can Taunt to prevent setup, roost to scout, or switch to a counter (easy to find given its weaknesses).
As a comparison Skarmory is less suited to balance teams, since it can't actually kill those he checks as easily as Mandibuzz.
He can't take special attacks as easily and he has less speed (can't Taunt as effectively).
However, on a pure physically defensive analysis, he can be considered better -> fits more on hyper offensive or defensive teams where pokemons are more one-sided.
Basically, if your "answer" can't assure you the upper hand in the current meta - permanent not a temporary check, then it's not really a good answer (for balance).
I consider that a direct switch to a teammate with great synergy is a good answer and can give the upper hand.
So the first step in building such a team appears to me as follow : a great defensive core.
Everything then adds up to it, to provide as much tools and synergy as possible. But it almost never starts with the offensive side as a basis.
Yet, the defensive core doesn't need to be perfectly self-sufficient.
But once you've built it, you should already have answers for almost every common OU threat.
Then, since there will always be holes, you should have at least checks to these other threats that functions as double-answers : usually one being more defensive, the other more offensive.
The offensive core, say a special sweeper, a physical sweeper and a wallbreaker (mixed or setup, very good coverage), usually consists of few "very good" candidates because of the synergy they have to provide with the defensive core. To alleviate their struggle dealing with common stall staples (gliscor, ferrothorn, chansey, skarmory, etc.), the defensive core should also be able to take care of at least 2 of them in my opinion. This way, against stall teams, since your defensive core is likely to have less endurance, it is able to cripple the opponent. And when you call for your offensive pokemons, you slowly add up damage to at least 2 of the opponent's pokemons, then you can capitalize on that to break through. It won't cut it though against extremely defensive teams, against which you stand almost no chance. I've always had far better results with balanced teams against HO than against stall.
On balanced teams, even your offensive pokemons usually are not almighty powerhouses because of the defensive side they need to provide. They just need to be efficient every time they come in, so an easy candidates is Greninja for instance : fast, immediate power, great coverage = fair chance to perform a -50% health every time you click. Plus, Protean can give welcomed resistances at times. Bisharp/Scizor are great priority users with utility (KnockOff, UTurn) and power. I tend to prefer Bisharp's coverage and speed though. Steel is a great defensive typing, and they muscle through most fairies - usually hard to beat with defensive pokemons since they are quite defensive as well : clerics, recovery, status inducing moves, boosting moves, etc.
The defensive core should have answers to Baton Pass teams, setup sweepers, not being overly weak to hazards nor status.
It should have means to slowly weaken the opponent : status, hazards, voltswitch/uturn, spammable attacks with good coverage (not necessarily strong power), etc. - special mention for lava plume, scald, gigadrain , this kind of things.
Therefore, certain moves are very useful for balanced teams : Roar, Taunt, SR, Spin, recovery, etc.
Other things are not really required : Boosting moves, Choice items, etc.
Concerning choiced items, as mentionned earlier in the topic, it's hard to put on balance since it gives the upper hand to the opponent. You can't act freely and use your versatility.
Priority is great on balance for that reason. It might be better to forego a fast sweeper with few defensive potential, for a slower one with priority and more defensive presence.
Balance is appealing in the sense that, if well built, it is often easier to change your gameplan and adapt during a battle.
You are solid enough to endure an unexpected incisive onslaught, while still retaining enough presence and living power to answer strongly next time.
You fail once, but don't lose. If you battle for some time and realise that your mistakes didn't prevent you from finding other solutions (not overly relying on XX% accuracy moves, or crits), then you might have a good balanced team.
Of course if you make a big mistake against a clever opponent, you're gonna loose anyway. But fate is not as easily settled as on HO in my opinion.
You shouldn't be stuck easily (and too soon) in a "i'm gonna just loose this pok" situation, since every pok fulfills multiple tasks.
In HO, it might be easier to think about it this way : "i'm gonna just call crusher n°2, crusher n°1 made holes the opponent won't have time to fill". With balance, you simply cannot.
Of course, this is my personal way of seeing balance. Others might see it in a more offensive way !