I'm curious for IA veterans to say what they think would be the most powerful possible IA party (lets say IA v5 for now) which is composed only of single-class and kits, no dual-class or multi-class characters.

Obviously this will not be the most powerful party possible, but I'm still curious if a decent IA party can be made of just single class characters (again, including kits).

Duplicates are allowed (e.g. 2 sorcerers, or 2 vagrants) if you think that it would be more powerful.

The most powerful is very difficult to judge. It depends a bit on what you prefer to see in your party. For IA I prefer 2 melee tasks, one thief task, one cleric task and 2 mage tasks. Added that some characters can sub for multiple tasks, there are quite a few possible combinations. Sometimes you can also settle for a druid rather than a cleric.

- Swashbuckler -> thief + melee
- Blade -> mage + melee
- Ranger -> melee + low end druid
- Paladin -> melee + low end cleric
- Auramaster -> druid + useful mage spells

Some ideas:
- Necromancer Prot. + Bezerker + Kensai + Swashbuckler + Sorceress + Cleric
- Vagrant Prot. + Bezerker + Assassin + Specialist Mage + Sorceress + Cleric
- Cavalier Prot. + Barbarian + Blade + Sorceress + Auramaster + Cleric
- Riskbreaker Prot. + Stalker + Skald + Swashbuckler + Specialist Mage + Cleric

Something nearly comparable to my last game (I took Riskbreaker Prot and swapped Vagrant for Assassin)
- Vagrant Prot. + Cavalier + Riskbreaker + Sorceress + Sorceress + Cleric of Talos -> forget about locks and traps. just use (minor) spell deflection and knock.

For Cleric kit I usually take Talos since you won't be using the offense very much in a single-classed character (Lathander or Helm).

If you are looking for the stongest single class party, forget thief, either kit or main class.
My opinion:
Vagrant protagonist
Berserker
Blade
Cleric (Any kit is good imho. Lathander gives some melee boost which is very good in IA. Helm gives fee True Seeing, Talos bonus offensive and defensive spell)
Sorcerer
Sorcerer or Necromancer