How do you decide your party composition ?

Easy. I usually play as a Male mage Hawke, so it goes something like this: Carver and Varric -and then everyone else. The only companion I never take anywhere, is Anders.

I tend to find ranged companions easier to use than melee ones. I need to control Isabela myself to get the best out of her, otherwise she has a tendency to die and not make the best use of her abilities. Fenris also seems to die quite a lot for me, though he can be built as a tank.

When I played through as a mage I found Aveline plus Varric and another mage was a good option, usually Merrill as I had creation spells, but otherwise Anders is probably better.

Varric all the time, same with Aveline. Bethany if I'm not a mage in act 1, Carver is always left behind. A strong case can be made for Anders as a healer is very useful, otherwise I pick an alternate mage/ rogue.

Well for me it depends on the playthrough largely with my heroes but that's the case with all these games. I tend based on what mission it is and also who I think will best compliment my characters abilities although usually one of the spots usually goes to the love interest and the sibling if they can be used

Full damage group works well, and so much fun. I like Anders in the group, but rather as damage, and Hawke mostly a mage (and I have a warrior reaver/berserker). With Fenris and a rogue or Merrill, this is destructive.

My party was mainly Hawke,Bethany, Anders, and Aveline. When Bethany was taken from me I used Merill in her place. So basically it's warrior, warrior/rogue and two mages. After I complete playing DA:O I will play DA2 a second time and switch the party, though Anders will always be a mainstay for me. The first time I played a warrior Hawke, and for the second time I will play again a warrior Hawke. Cause I can't imagine doing that Arishok battle as a mage or rogue.

Hmm,  I did not think of it that way.  The problem I'll have with playing mage is losing Bethany early on.  But then again I could always level up Hawke the same way I leveled up Bethany.  Choosing the same skill trees and play style.  I'll see when I start a new game.  As for rogues, I normally am bad when playing rogues in any game.  Can't never get used to the idea of backstabbing the enemy in combat.  I always approach them as a warrior.

That reminds me I have to uninstall/reinstall DA2, cause when I completed my first and only playthrough I never had a marbari.  I assume it's part of a dlc?  Dlc's seem not to be working in my game.  I have to try to fix that before I play.

Mage and rogue are both insane against the Arishok. As mage, you can stun him with a lot of spells, as rogue, you can use decoy and he will attack the false Hawke. Both great fun

I try to have everyone in the party, apart from Sebastian, so I switch a lot. Anders is a good mage companion, when I play as a rogue. Aveline and Fenris are both good as warriors and don't get me started on Varric and Isabella, they just rock (in every aspect). I think, both siblings are good and I use them as often as I can.
Merrill stays at home the most (apart from Seb), simply because I like two rogues and a warrior and Anders and Bethany are the better mages imo.
I even gave Merrill the Spirit Healer tree with a mod, but it feels kind of wrong.
She does a lot of damage though

My main group is in the Act1: Carver, Anders and Fenris. I switch Fenris and Anders sometimes with the others, Carver only rarely stays at home. In Act2: still Fenris and Anders are the core, but the third change more. But I already know all the party banters...

But I play on normal-easy, so: the party setup no matter, it's just for fun and RP.

I like a two-Mage party with Mage Hawke, Merril, Isabella and Aveline. There's very little that this setup can't handle. Aveline tanks, Isabella provides extreme single target damage output, while Hawke and Merril provide AOE cover, crowd control, healing, debuffing and utility. Isabella in particular is an absolute monster at higher levels. I usually end up with her critical strike damage completely off the chart; she's like a human buzz saw! I just used her to burn down Meredith quickly during my last playthrough while the rest of the party kept the statues occupied.

This line-up also has a great group synergy in terms of friendship and banter. Isabella and Aveline can initially be a bit snarky with each other, but the banter becomes increasingly good natured as the game progresses. I don't have any particular story-related or lore reasons to use this party, it's based purely upon versatility and that nice group dynamic.

There's times when I have no choice but to swap Isabella for Varric (such as The Deep Roads, end of Act 2 and Legacy), and obviously Talis takes Isabella's place for Mark of the Assassin, but on the whole the above is very much my core team, my posse of choice.

Conversely, if you want a pure hate fest, use Anders, Fenris and Merril. The bad vibes are palpable.