Sera and Cole. Sera because she was so incredibly annoying, she's probably the only character I've ever felt tempted to kick out of the group. Cole beacause he died so easily, otherwise I really liked him. I'm going to spec him as an archer in my new playthough and see if that helps.
Vivienne and Blackwall got to come along a little more but weren't used that often. Blackwall is a little bit boring before the "reveal" and Vivienne died very easily before she got her spec and after she got it I already had favourits. Iron bull got to join the party a little more. These three were my go-to team for dragon fights though.
I used Solas a lot in the beginning because it took a little while before I left the Hinterlands and recruited the others. Because my character was a mage I never had more than one companion mage with me (need warriors for tanking and bashing in walls, and rogue's for opening locks) and once I recruited Dorian he became my main mage. I decided I would bring Solas more on the playthrough that I would romance him in.
Cassandra, Varric and Dorian were the ones I used the most. I just really liked them.