Hmm, my favourite companions are probably
DAO
Hard to pick between her and Morrigan, but Leliana. I love that you can turn around, ask her if she has any stories, and she can spin off tale after tale. Talking to her is so rewarding. She's sympathetic once you learn of her past, and, while Morrigan's banter with Alistair is hilarious, sometimes Morrigan is too much of a cynical ******, even for me.
DA2
Varric, obviously.
DAI
Probably Dorian. I didn't like him at first, but his recruitment mission is maybe the best mission in the whole game. He's caring, but not a simp. He's talented, but not arrogant (and when he is, he's usually self-aware). He really gives us an insight into Tevinter, shows that not all Vints are bad, and he is also genuinely quite charming. And dat moustache.
I think my worst would be:
DAO
Sten. He's so grumpy, and his character model stands out as particularly ugly, in a world where even the prettiest girl in Orlais is ugly. Also, dude, maybe don't throw a tantrum and murder a bunch of innocents because you lost your sword, mmkay?
DA2
Probably Anders. both he and Fenris are a pair of whiny, angsty idealogues who never shut up and act like teenagers. What seals the deal for Anders though is that he's an insane terrorist. At least Fenris only went after the people responsible for slavery.
DAI
Vivienne. Her selfishness really comes through in very ugly ways, especially when her lover dies. She thinks she's better than everyone else, when she only really got to where she is by sleeping with the right guy. She never really uses her diplomatic skills to help the Inquisition, she's leader of the circle, but we never see her lead anyone, and, to be honest, Solas (knows about the fade) and Dorian (time travel magic) outclass her as mages by a country mile. And she reminds me of many people in the real world, who defend broken, unjust systems because they managed to benefit from it, completely ignorant, apathetic, or both, to how others less fortunate struggle.