I've done all except Solas and Bull. Bull I'm really not interested in as a character and from what I've heard, the majority of the romance is actually just talking about BDSM and how Bull knows what the Inquisitor wants better than the Inquisitor. The fact that people like this attitude reminds me why 50 Shades of Grey is popular.
Solas I just really had no interest in, he's an interesting enough character, but I honestly resented the fact that the game effectively gates some of the relationship between him and the Inquisitor to female elves only. Morrigan had a nice friendship track for female Wardens and male Wardens not romancing her, so the DR plot could still be emotional to someone not romancing her. Anders could still be a close friend or a bitter rival if not romanced. Solas can still be friends with the Inquisitor, yes, but I never felt he was that close to the PC. It was more of a professional respect, or at least as much as he's capable of providing to someone who's not an ancient elf. Only a romanced Lavellan gets a true emotional connection to him, which waters down the impact for me.
My canon romance is Cullen, as he's a fairly easygoing romance but it does have some angst in it. He's tied to the plotline with Samson/red templars and is an integral member of the Inquisition. He's committed to the cause and isn't there for his own benefit. Plus my canon Inquisitor is a faithful Andrastean mage which makes for an interesting pairing. And with the wedding in Trespasser, he officially becomes a partner to the Inquisitor in their mission.
I liked the Cassandra romance as well. She's one of my favorite DAI characters, which helps. I liked that trying all of the nice or flirtatious dialogue options didn't always net approval from her. She's true to herself and doesn't put up with empty words. And it's nice to see a more romantic side to her, but it's not romance gated since we still get hint of it through her book scene with Varric.
I liked Sera because I played as a Qunari Inquisitor. I haven't tried to romance her as an elf or mage, otherwise I probably wouldn't have enjoyed it. She's a fun character but really requires the PC to agree with everything or else back down quickly. She's not someone who tolerates opposing opinions.
Dorian was okay, my first playthrough romance. It was a bit melodramatic as so much of it revolved around gayngst, IMO, and he hoped for love and not just sex but wasn't going to put his heart on his sleeve. I like him as a character but too much of his content was an early 2000's gay coming out film. I've seen plenty of those in real life and don't feel a strong need to play them out in my games.
Josephine was good and prior to Cullen was going to be my canon romance, for the same reasons Cullen has. She's tied to the Inquisition, believes in the mission, and is always there for us. But her content is a little short, the duel is a little silly, if entertaining. And honestly while I like her character, there's not tons of depth to her. Plus no matter what it seems like she leaves the Inquisition at the end of Trespasser. The DLC indicated that she had grown disillusioned of its mission or felt that it was time to step down.
Blackwall's romance was about as interesting as the rest of him. I.e. not much. I had already played his personal quest so knew the twist, but the romance didn't really seem to add that much to the reveal. I kept waiting to get a better sense of him as a person and not a GW but never did. I actually liked him less after it due to some of his actions, which are pretty cold to someone he says he is in love with.