One of the most important things in Pillars of Eternity is the correct picking of your party members. Rooster of your team will change many times during game. You will meet new companions and want to include them in your party, so lots of choices await you. Below you will find an example of a perfect team - maximally versatile and useful in every situation.
Fighter or paladin with a shield is absolutely necessary in your party. At least one character should be focused on taking damage from enemies. All his stats, abilities and skills should be focused on defense. Such character will be almost indestructible and other party members will be able to concentrate on dealing damage. You should have another melee fighter in your time, but focused on dealing damage. She should be able to block enemies as well, so that they won't target your shooters. Barbarian with two-handed weapon is a good choice for that.
Warriors should have support from distance. Because of that, one of your companions should be a ranger. First of all, he has longest reach, so he can safely attack even most far away enemies, and he has an animal companion that will support other warriors. Next are characters that are 100% focused on dealing damage. Wizard and a druid will be best choices. They have very strong spells, and a combined attack of both those classes in one point will end with quick death of enemies.
In that way you have a five-member party, but you still have one free slot. You have two options to fill that place - pick a priest that will heal and help his companions, or chanter. Second option seems to be better as chants have long range, greatly improve your companions and the character is able to use a strong spell from time to time. Additionally, you can equip chanter with strong armor or shield and he will be able not only to support the team, but help paladin or fighter in blocking enemies. It will improve his endurance as well, so he will be very effective all the time.
In summary, "dream team" should consist of six characters: a paladin, barbarian, chanter, hunter, wizard and a druid. It will be a versatile team that will work in every situation with any opponent it will face.