Well for Inquisition it feels like Solas, and Cassandra often belong the most or have the most to say (honestly Cassandra feels like the main character half of the time, which i really like). But my suggestions with what I have played would be this:

In Hushed Whispers - Cassandra and Vivienne, then the last spot I would fill with either Iron Bull or maybe Solas.

Champions of the Just - Again Cassandra and Vivienne (these first quests relate to the mage and templar war and these two have the most involvement in it) then for the third spot I might suggest Varric, the rest don't seem to say much from what I have seen so far.

In Your Heart Shall Burn - Doesn't really matter, nothing really changes no matter who you bring. But I personally like to bring Iron Bull, Sera, and either Dorian or Blackwall (the more "expendable" sort, for RP purposes.)

Wicked Eyes and Wicked Hearts - This is mostly a toss up between everyone. It really depends on who you like. I like to bring Cassandra again for RP purposes and how important she feels to the story. Vivienne is good for her experience in Orlesian politics. Blackwall gets some interesting nods that make sense later. Sera has some pretty funny stuff. Varric is well admired by the court and has many fans. And Dorian kinda fits too because of his experience in Tevinter politics. Solas is amused by the whole thing, and I think it just makes Bull really uncomfortable. Cole is mostly ignored.

Here Lies the Abyss - Would recommend Cassandra, Solas, and Blackwall. Maybe swap Blackwall with Cole if you don't care about Grey Warden stuff really. Or Varric works nicely too.

What Pride had Wrought - Solas, Cassandra, Varric. Solas especially. But if you want to swap people out I would say maybe bring Dorian or Sera, particularly if you are romancing them.

Doom Upon All the World - Anyone really. I think there is a couple specific lines of dialogue to everyone.

The rest of the main story missions don't let you choose or have a party I think