On Myrth
I actually haven't tried to romance Myrth with Saren (Rei and Myrth are more of the bishounen/bishoujo type characters I prefer to go after in BL and yuri games when it comes to deciding to romance characters on aesthetics alone--which I frequently and unabashedly do

--though I did have Elenor romance Rei because I realized he could be pretty cute despite being such a blood knight, haha), but for what it's worth, I thought that Myrth and Saren's friendlier banters whenever I chose "Joking" was some of the best; the way Myrth indulges in Saren's humor every once in a while when most of the characters either ignore it/are confused by it/are irritated by it/are oblivious to it and can be lighthearted with him that way was what I really liked about their interaction.
I probably should play through Saren's romance with Myrth just to see why people weren't as fond of it as others (planning on doing so right now), but I think if people don't like it because their romance has the "beautiful woman looked/chased after by protective young man who thinks everything she does is kind of wondrous" relationship archetype (I might be wrong, just going off what I'm reading by the people who actually voiced a reason and general remembrance of Saren's thoughts about her while non-romanced), it might be that that kind of relationship has been almost too often used in standard fantasy anything (novels, manga, games, comics).
But that makes me wonder: does she have a badass moment in the romance? One of my favorite romance-related parts ever in Loren was staying with the party at the end in Draco's route, since he basically flings himself away from the party to protect everyone whilst trying to avert their minds from worrying about him by being casual about it -- maybe something cool like that might endear people to her. (In fact, during my first playthrough I didn't stay, and doing so in my second playthrough with the route cemented how much I loved Saren and Draco's romance.) Personally I like romances built up on friendship or rivalry (where they grow as friends or rivals before the romance really takes off), or equal relationships (where both depend on each other rather than just one character depending on another), and if, even in unequal relationships, the person who depends on the romancing/romanced character comes to depend on himself/herself at some point, at least. It's not just in romances either; I think that what a lot of people prefer in fiction is the bravery and the success of the bravery of characters, whether they're masculine or feminine or both or neutral. Just my two cents after watching/reading/playing through thousands(?) of romances over the years, if any of that helps.
Anyway, one of my favorite pair of friends was Saren (and along those lines Elenor) and Myrth because of her ability to take a joking Saren in stride (her confronting Saren about Mesphit if you let him die by echoing sentiments passed onto her from Saren that she'd learned and grown from over the course of the game was also a plus), and I don't mean that in a disclaiming "I like them just as friends" way either; I honestly liked their friendship after Myrth became less reserved around Saren, since they were pretty companionable to each other from what I recall (there was one point where joking!Saren and she made light jabs about her vine-attire, although I don't remember exactly what they said right now--like that time Loren came back with a joke of her own to joking!Saren/Elenor, however, it was fun).
On my favorite romance (etc.)
All of THAT being said, It's probably obvious that Draco's romance was my favorite, from my momentary gushing. I just really, really love the "seemingly carefree and comedic--but always loyal and trusting (even when he hides secrets)--best friend who's secretly in love with the MC" type of BxB romance (Lloyd/Zelos type

, except Zelos didn't have a crush, probably), but I'm rarely indulged! I was so delighted when I found out I could actually romance him, and promptly did, and wasn't disappointed at all. I've romanced him in 3 playthroughs--practically everything about it hits my "endeared" points, from their warm, growing friendship, to their awkwardness in approaching their crush (Draco)/possible reciprocation (Saren), to Draco's hiding his own insecurities so that he doesn't worry anyone but failing around Saren, to his moment of being badass, to how very sweet it is. I could fangirl on about it, I like it that much; kudos to the writer, yet again. The artist and colorist made his expressions quite adorable, too, for the lack of CGs.
I love warm, glowing friendships, haha. I did say I liked rivalry-to-romance too, though, and I thought that PC/Amukiki's was electrically-charged and made one (i.e. me) eager to follow in Amukiki's route just to see when it would hit its peak and turn to love

, which is the highest praise I can think to give about belligerent rivalries turning into concerned romance. Also, Saren and Amukiki's CG was the hottest both written and drawn, imo, after going through a bunch of them.
On Mesphit, despite his being so popular (brooding and passionate bishounen with dark pasts and huge problems take the cake), I think I'm past my days of adoring the "determined MC takes the time to understand and help the broody (and sometimes destructive) possible love interest with a dark past" type of romance...is what I would say if I wasn't totally head-over-heels with Hawke and Fenris' romance in DA2.

But that's through the (DA2) friendship route, though; I think one of my problems, despite liking the Mesphit romance nonetheless, was that their romantic encounters were based more on passion, i.e. the protagonist (literally and figuratively) chasing after Mesphit while Mesphit struggled against them (and his demonic side, of course), than a steadily growing warmth and camaraderie in their relationship. That might just be a personal preference, though, since a lot of people obviously like it for exactly that.

I wouldn't say no to seeing more camaraderie (since they got through their problems together) along with passion between the MC and Mesphit in the next game, though!
This got quite long (sorry!), so I think I'll leave it at that.