A caveat: I'd rank pretty much all of these characters above the vast majority of the DA 2 companions, so in general, I like them all.
My Top 3 (also probably in my top 5 DA companions of all time)
1.) Vivienne - she's a brilliantly written character; incredibly complex and Indira Varma killed it as her VO; she might be my fav companion of all time, to be honest
2.) Dorian - charming, dashing, and so arrogant I can't decide if I want to punch him or kiss him most of the time (spoiler: kiss him); I think he's Gaider's best character
3.) Cassandra - I thought she was going to be one-note and boy was I wrong; I love the little bits of vulnerability that she lets you see; she's my must have 'bro' for every major quest in this game
My second tier (really interesting and enjoyable in their own right)
4.) Varric - he was my must have 'bro' for DA 2, so it's interesting to see him in a different light; his author did a great job of showing another perspective on an already well-established character
5.) Blackwall - he's a little dull for most of the game, but I think his companion quest is one of the most interesting ones in any Bioware game; didn't quite have the pay-off I was hoping for, but still a great ride while it lasted
6.) Iron Bull - he's a little one-note for me; he's over the top "bro!" with his obsession with fighting and sex; but he's a nice guy in the romance and I do like his relationship with the Chargers
My Bottom 3 (I don't dislike any of them, but they are the worst of the bunch for me)
7.) Solas - dull, dull, dull; yes, it gets more interesting post-game; but in-game, I just can't figure out a way for he and my Inquisitor to get along; I'm just not a fan
8.) Sera - she's a little annoying and immature; her pranking quest was a waste of my time; but I do love her accent
9.) Cole - just kind of a dud; maybe it's because I never read Asunder, but I just can't bring myself to care about him. At all.