I despised, and continue to despise, Sera. She's way, way too much of a try-hard "funny/random" character and her actual motivation for joining the Inquisition is... did she even have one?
I originally hated Vivienne however she grew on me greatly over time and now I think her look, manner of speaking, and writing are all superbly done. She also brought up a lot of compelling arguments against getting rid of the Circle system for mages.
Blackwall is sort of... meh. He's obsessed with the Grey Wardens to the point of it being annoying, but I can't say whether that was intentional or not as
I love his beard though.
Aside from that, they're all pretty damn good. Cole is fantastic, the little pop culture references he constantly says that seem cryptic and weird at first are awesome to hear and figure out. Dorian is an all-round likeable character to talk with and have in your party. The Iron Bull is charismatic and hilarious. Solas is a very intelligent, cool-headed member that seems to have knowledge on any subject you could think of. I ended up liking Cassandra much more than I thought I would, especially as basically the only real romance option available for straight male characters; her character development and revealed "guilty pleasures" quest with Varric was cute and excellently done. And of course, there's Varric, IMO the best companion in DA2 by a massive stretch, not really possible to dislike him.