In DA2 the most powerful party consisted of 3 mages and a Rogue (Varric, Anders, Merril and MageHawke). Everything was super easy, with the exception of the Alpha Wyvern, which was tedious, cause you had to pause and move your targeted teammates out of the way, 2 fix-in-place spells(Glyph of Paralysis and Gravitic Ring) plus 3 Firestorms, 2 Tempests, 2 Chain Lightnings and a Walking Bomb destroy everything, fast. Not to mention Varric making short work of dangerous Lieutenants by CCCs.
In DAO the most powerful party was also an all ranged party - 3 mages and Leliana. The PC gets to do Storm of the Century and Mana Clash, Morrigan gets to do CC and Wynne heals, buffs with Haste, Mana regen, etc. However, the most easy-going party was two 2-handed warriors double hasted by Morrigan and Wynne (once you remove the Haste bug by using the PC combat fix). Everything dies incredibly fast to auto-attack. You just have to put some points toward disabling enemy mages. I've also tried 4 Warriors and while easy, it is not as potent.
As long as you have a methoid of controlling enemies, sticking them in a place for a certain amount of time, I've no doubt the most powerful party in DAI will again be a combination of ranged Nuke and AOE abilities. So, probably three teammates from among Sera, Viviene, Dorian, Solas and a Mage Inquisitor.
Which won't stop me from choosing a class that lets me and Cass do a metric f*ckton of damage via CCC and having her always in the party. 