In terms of fighting battles and mopping the floor with the opposition, I was pretty fond of my final party from my Sereda Aeducan DA:O game -- dual-wield stealth Assassin rogue Aeducan, Templar/Champion/tank Alistair, damage mage Morrigan, support mage Wynne. Our default party in my first time through DA2 -- dual-wield rogue Marian, archer Varric, damage-and-tank Aveline, and support mage Anders -- worked really well, too.
If you're asking with whom I most enjoyed running around Thedas, it has to be the ladies of Dragon Age 2: Isabela, Merrill, and Aveline. Because Isabela largely duplicated my Marian Hawke in terms of battle skills, I didn't get to pay with this team much the first time through, but they were a great, well-balanced group in Marissa Hawke's game, and so they saw a lot of action.
But my favorite thing to do, really, was to switch people in and out as much as possible, so I could hear all the banter and get all the interactions. Every team is different, every group has its own charms. So if you're asking for my favorite party, the answer is: all of them!
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