A classic and effective party composition could be a tank, a healer, a ranged damage dealer, and a melee damage dealer. For example, a Fighter as the tank to soak up damage, a Cleric for healing, a Ranger for ranged attacks, and a Rogue for melee damage and sneaking around. This allows for a balanced approach in combat and different skills to be utilized during the story quests.
A composition with a Ranger, a Cleric, a Monk, and a Warlock. The Ranger can be a ranged attacker and tracker, good for exploring and dealing damage from afar. The Cleric is essential for healing and also has some buffs and debuffs. The Monk is a unique melee combatant with high mobility and unarmed attacks. The Warlock has some interesting spellcasting abilities and can make pacts for different powers, which adds a lot of variety to the party's capabilities in the story.
The best party for BG3's story could be a Barbarian, a Druid, and a Monk. The Barbarian is great for raw combat power and can soak up damage. The Druid can transform and has a variety of spells for different scenarios. The Monk is fast and has unique combat abilities that can help in battles throughout the story.
A party with a Cleric, a Rogue, and a Wizard can be great for the story in BG3. The Cleric can heal and provide support, the Rogue for sneaking and lockpicking, and the Wizard for powerful magic to deal with various challenges in the story.
Astarion is a great choice. His complex backstory adds depth to the story. He's a vampire spawn with his own motives and development throughout the game. Another good one is Shadowheart. Her mysterious past and her role within the Sharran cult bring interesting story elements. And Gale, his connection to the weave and his pursuit of magic in a rather dangerous way enriches the overall story experience.
A good party composition could be a Warrior for front - line combat, a Ranger for ranged damage and some crowd control, and a Mage for powerful spells. The Warrior can tank and deal melee damage, the Ranger can pick off enemies from a distance and slow them down with snares, and the Mage can use area - of - effect spells to deal with groups of foes.
For story, I'd recommend Shadowheart. Her loyalty to Shar and the slow reveal of her true self as the story progresses is fascinating. Then there's Astarion. His journey from being a subservient vampire spawn to potentially finding freedom is a great story arc. Karlach is also a good pick. Her bubbly personality combined with her tragic backstory of being in Avernus makes for a very engaging story element in the party.
I think a party consisting of a Monk, a Chanter, and a Priest is great for the story in Pillars of Eternity. The Monk is a very unique combatant that can add an interesting dynamic to battles in the story. The Chanter can use their chants to buff the party and also has some story - relevant lore. The Priest is essential for keeping everyone healthy during the story - driven quests.
Gale is another excellent choice. His pursuit of magic and the mystery surrounding his condition with the Netherese orb. His relationship with Mystra and his attempts to regain his former status contribute to the story in many ways. He also has a lot of knowledge about the lore, which enriches the story as you progress.
In the BG3 story, some of the best companions are Shadowheart. She has a mysterious past and her own beliefs that add depth to the story. Another great one is Gale. His knowledge of magic and his personal quest for power and redemption make him an interesting character to have around.