I think I'd probably have to plump for the cast of DA:O, and not just because I've got a fangirl thing going for Alistair. Alight, maybe that influenced me just a little bit.
Anyway. I appreciated the range of characters available in both games, and the fact that they each had their own stories and motivations, responding to different things. It was realistic, in that not everyone was always going to agree with the choices I made. It forced me to think about what I was doing and who was with me.
One thing DA:O has over the sequel for me is that I could talk to my party whenever I wanted. I could be wandering round Denerim and strike up conversation with whoever was with me. OK, so that occasionally ended up extremely inappropriately, but it was good. DA2's method of only being allowed to advance character stories and conversations when the game told me felt restrictive.
The banters are one of the things that keep me coming back to the game. Alistair / Morrigan sniping at each other, Zevran / Wynne, Oghren / Alistair... The interaction between them was great. I particularly enjoyed the Aveline / Isabella dynamic in DA2, as it felt like their relationship advanced. Aveline's "whore" comments begin as quite cold and nasty, but by the end, it's more in the nature of friends taking the mickey. Just my impression, anyway.
In DA2, I did miss the Anders of Awakening, and his mocking humour. I see why he'd changed, and it was an interesting idea, but I still prefer old Anders to moody, self-flagellating Anders. Varric was clearly awesome. Fenris was... Very angry. Again, I can see he'd have a reason for it, and Gideon Emery's voice was enough to make me forgive a lot in that instance. But I'd have liked to see more development, perhaps?
DA2 did have a good idea with the friendship / rivalry thing though. I liked that the cast would still hang around with you, even if you disagreed on things, they'd just have a different kind of relationship with you.
I've played Origins more, so this may well have a bearing on everything. Maybe I've bonded with these guys because I've spent more time with them.