Best:
Aveline
Varric
Shale
Dorian
Cassandra
Blackwall when not talking about himself
Cole
Sten
Nathaniel
Loghain from Inquisition
Worst:
Marethari and Orsino, for both making 0 sense and having irrelevant personalities and presences before act 3.
Anders, too whiney
Fenris, too angsty
Morrigan in DAO, nothing but a ****** through and through unless romanced.
Flemeth, became a literal Deus Ex Machina
Solas, Spirits deserve Equal Rights
Sera, just Sera.
Blackwall whenever he talks about himself.
Fiona, mary sue and turns a blind eye to atrocities in books, incompetent fool and turns a blind eye to atrocities in games.
Best of the Best:
DAI Morrigan after romance and child. The entire character arc, with her and the warden, and beyond, made her quite the endearing character to me. Her actions make sense in a way, and the character evolution going through this route not only felt natural, but also superior to others, say Leiliana.
Worst of the Worst:
Meredith. You had a character with a lot of potential, one who showed to be a multifaceted character. Harsh and hard but also honorable and good intentioned, and it was more her attempt to make her good intentions achieved that caused problems. So what do you do? Why, strip away her complexity bit by bit until there's nothing left but a delusional dictator with a magic sword. Oh, and you do it through a timeskip and never let the player actually see meredith or interact with her outside of once in Act 2 before getting the bulk of impression from her when she's already half-crazed with paranoia.
Wasted potential is worse than bad potential, so Meredith is worst of the worst.