Hey now no one said the word old 
I'm just sayin the female Li's are traditionally the most popular and Dorian well you've seen his thread right. Cullen like I said is a lot like Garrus he gets **** done has a few demons and is a bit shy awkward when it comes to romance. Solas if he's a romance will probably be bisexual and you know how much people love bi elves. So that leaves IB and Blackwall (I forgot Varric) out of the two of them Iron Bull seems like the type that some people might romance just to say they did which is why I put him over Blackwall. That's my reasoning and this is assuming Viv, Solas, Varric and Blackwall are even LI's and Blackwall as well as Varric are low on my list of expectations.
Yes, my bad
You know what I meant though!
I wouldn't judge a character's popularity based on his/her thread. You have to first ask how many people posting in the thread intend to make Dorian their first playthrough romance. Then count the number of people regularly posting in the thread. Consider that Dorian is a m/m romance; this will wildly cut down on his popularity since I think most DA players are straight males. (DG has said that 30% of players made a female Warden in DAO, so that's what I'm basing this on.) And lastly, remember that online popularity does not always mimic in-game popularity: Leliana was the most popular DAO LI over "fan favorite" Morrigan.
I personally think Dorian's sex and sexuality will place him at the bottom of the list. BioWare attracts a LOT of gay fans, understandably so, but even so, I don't think it's enough to propel him above straight and bi options. Yes, some straight and bi women will eventually romance Dorian, but only after they've done Cullen or [insert male LI of choice].
I think saying people "love bi elves" is, erm... anecdotal evidence to say the least? All the elven LI so far have been bi. Zevran was bi, and that still didn't make him even close to the most popular LI. You also have to take into account that Solas, erm, doesn't exactly look like pretty boy Zevran or smoldering broody boy Fenris. Sure, he has his fans, and his story/bio seem REALLY interesting, but again, not enough to propel him to the top.
Iron Bull: He is a bi romance, so that's going to be in his favor. BUT he's an odd-looking dude, not exactly GQ cover material. He's a muscular fella, so he'll pull in the gay/bi male crowd, but again, that's a very small subset. He's bi, so he'll attract some female players... but his competition for straight females is Cullen.
Varric: Too many variables to even guess-timate. He's well-known and well-loved, and rather good-looking, but he might be race-gated as well as gender-gated. Those are going to cut down on his popularity.
Sera would have rocketed to #1 if she'd been bi; she'd be the new Leliana I think. But being f/f only, she's the female Dorian... lesbian and bi female fans are no doubt excited, but that's a small pool of the overall player base. A few straight males will play a female PC to romance her.
Cass will likely be #1; it's between her and Josephine for me. Josie is an NPC, but she's bi, allowing her to scoop up more of the player base. But the majority of fans (again, assuming) are straight male, and Cass is a companion. That might be enough for the #1 spot.
Cullen is a handsome human male, and his straight-ness will probably not hurt him too badly, similar to Alistair. But his NPC status could. If so... his biggest competition could be Solas/Blackwall, and I think those two are so very different that it's impossible to guess which would be more popular with straight women.
I've now spent a stupidly long time thinking about this and typing it up
But my current research has me looking at demographics, so I guess that's why it piqued my interest.