RE: So just out of curiosity, how do you decide...?
For me a good team has always been one that keeps the momentum on your side, a team that has offensive synergy and doesn't give your opponents that many chances to switch in, while you have 2 checks, checks not counters, to every common pokemon in the metagame. I maintain momentum by simply using a lot of choice specs and choice band, and try to handle the common sweeper in the metagame by having a lot of priority and fast pokemon. I never liked having scarf that much to check something because if it has some boosts in speed, or is a faster scarfer you're screwed. Hazards, speed and offensive Coverage, paired with Priority and good checks to common sweeper is what makes a good team for me! But this is simply my playing style. A lot of people tell me that it is wrong to have 4+ Choicer on every team, but as soon as I try something different I lose against every a lil' bit better made team. The only other strategy I am somehow capable of playing with is stall, but making stallteams takes a lot of thinking, like really a lot. Which team is good and which team sucks, what makes a team good and what is totally useless and wrong on a team depends a lot on the player using the team and on his mindset. When I use stall I totally have to change my mindset, I cannot simply let a Pokemon get down to low health like I can do it with my sweeper when I want to handle something, so I need a safe switch in for everything, every single Pokemon in the metagame. But one thing I can surely tell you is that every good team, doesn't matter who made it will have good synergy, either offensive or defensive. One thing I think is nessecary on every team are rocks, since the free 12.5 % already make 125 & health in 10 turns, which is a lot.
I can tell you how I make my teams, I cannot be sure if it will work for you, but you can try, it won't kill you. Btw I always use paper and pen to make my teams, for notes otherwise I cannot make a good one.
First of all I choose a sweeper, one Pokemon to base the team around and to sweep with in Lategame. Then I write down a list with Pokemon that stop it, and with Pokemon that can kill it once they come in. Then I try to find something that lures the counters and checks. This is usually the most difficult part for me, to find out how to kill a counter without letting it switch out immediately. This is where either my creativity or simply a choice band comes into play. Then I try to have somthing to handle all the Pokemon that can kill my pokemon once they are brought in. Then I simply try to minimize the chances for my opponent to have something that sweeps me. I usually need 2-4 Pokemon for this. Then I try to have hazards on my team, and for me it is usually either Froslass or in lower tiers Glalie. Then I try to have something that gives me more offensive synergy that forces switches. I never really spend any thoughts on stall because there is nothing that really breaks stall, you have to play good against it, because every good stallteam will be able to handle the stallbreakers you have. My teams are usually offensive, I am not someone who can win with 4 bulky sweeper and 2 walls or whatever. Either stall or all out offense and hazards for everything.
But see, this is what works for me. It all depends on your mindset, do you prefer being safe or do you like to get the opportunity to kill Pokemon at the switch in, do you like a team that can has a safe switch in for everything or do you prefer something that makes it harder for the opponent to switch in but that requires playing around some threats. I don't think there is a special key to a good team, but it all depends on preferences and mindset. The only thing I can tell you every team needs is synergy.
Just go out and make your team by playing and testing,
