The thing about Overwatch is that there is no 'best team' to build, but there are key points every team needs, and not having them can spell defeat right from the hero select screen. Even more so if the people playing are too rigid in their choices to switch to whatever it is that the team does need, when they're playing something that really brings nothing to the table for the team.
Step 1)You need tanks and healers.
If you are missing one or both of these roles on your team, you are almost certainly going to lose. It doesn't matter how much damage your team has at that point, you've got no one who can stand on an objective, and if you get wounded you have to fall back to find a health pack rather than stay in the fight with a healer keeping you up. Ideally, you want two tanks for most maps, one to disrupt the enemy, and one to protect your team. Generally, only one healer is required, but if you are on defense then you might consider a Mercy and Lucio team up. With them combined even the squishiest offense characters will be hard to bring down.
Step 2)Don't duplicate heroes.
By playing two of the same hero, you are bringing nothing new to the table for your team than the other person isn't already bringing, While you may be good at that hero, it doesn't generally help to force duplicate heroes on your team. If the other person doesn't switch, then switch yourself and bring something new that will be more beneficial.
Step 3)Adapt to the other team.
This is the most important step, and is why there is no 'best team' for Overwatch. You can go to your spawn point at any time during the game and switch to a new hero. If there's one hero on the enemy team that is giving your entire team a hard time, switch to something that can deal with them so the rest of your team can do what they are supposed to be doing.
Most importantly, never count your team out. You can always turn things around by switching to a new team composition mid game if you need to.
Step 1)You need tanks and healers.
If you are missing one or both of these roles on your team, you are almost certainly going to lose. It doesn't matter how much damage your team has at that point, you've got no one who can stand on an objective, and if you get wounded you have to fall back to find a health pack rather than stay in the fight with a healer keeping you up. Ideally, you want two tanks for most maps, one to disrupt the enemy, and one to protect your team. Generally, only one healer is required, but if you are on defense then you might consider a Mercy and Lucio team up. With them combined even the squishiest offense characters will be hard to bring down.
Step 2)Don't duplicate heroes.
By playing two of the same hero, you are bringing nothing new to the table for your team than the other person isn't already bringing, While you may be good at that hero, it doesn't generally help to force duplicate heroes on your team. If the other person doesn't switch, then switch yourself and bring something new that will be more beneficial.
Step 3)Adapt to the other team.
This is the most important step, and is why there is no 'best team' for Overwatch. You can go to your spawn point at any time during the game and switch to a new hero. If there's one hero on the enemy team that is giving your entire team a hard time, switch to something that can deal with them so the rest of your team can do what they are supposed to be doing.
Most importantly, never count your team out. You can always turn things around by switching to a new team composition mid game if you need to.