I'll try to pick one per game.
Zevran: I didn't like him at first, thought he was an untrustworthy sleazebag. But the more I took him in my party and spoke to him, the more I realised he often made me laugh and feel happier. I really like his combination of easy-going humour and cynicism. He's got a horrible past, and some unscrupulous methods, but still has a good heart. He objects to almost all of the more cruel choices, and particularly hates it if you harm children or elves. Hell, if you let the demon in Honnleath get Amalia, he gives more disapproval than Alistair. I really enjoy his romance too, because of the focus on building up trust and friendship.
Isabela: Similar to Zev (since I never learn!), I made snap-judgements before she joined and didn't really bother with her. So she ran off. I invested time in getting to know her on my second PT, and learned that not only does she have the good sides of Zevran (that light-heartedness, humour), she's really kind. Her interactions with Merrill are just pure gold, and she has such strong feelings on what's right, even if she tries to brush it off. I was also surprised at just how wise she can be (such as in her banter with Anders about the concept of justice). Her self-confidence, acceptance and focus on the present (rather than the past) is really quite inspiring too.
Cole: This is a tough one because I haven't played DAI as often, so I haven't settled on a definite favourite. Cole is perhaps tied with Cass. I really love him though, because he's compassion personified. He wants to help people, offers rich insights into lots of characters and places. He's had such a struggle from Asunder through to DAI and his character can take two very distinct paths that emphasise different aspects of his nature. I find spirits fascinating anyway, so it's amazing to learn more about him. I like finding out about the 'strange things' he's done in Skyhold, even his manner of speech is very endearing.
Edit: I will say that Cass is a prime example of where I finally learned to have an open mind towards characters I'm iffy about, instead of leaping to conclusions. And it really paid off, I love her!