The first time I read this was almost 10 years ago. Since it looks like we're finally reaching the end, I reread the whole thing recently and edited it now to reflect my current opinions, but I still feel generally the same way about it.
My favorite part of this series is how the two main characters actually truly love each other. It's not often that a manga can convey this much emotion, but their love is so pure, so desperate, so real...
Then there's the characters being so likable.
Takano is exactly what I like in a seme. Yes, he's pushy, but it's understandable. He's been in love with Ritsu for 10 years and had no idea what happened between them was just a misunderstanding. Even though Ritsu went through the same experience, it's different for Takano. Imagine how desperate he must be, being so close to getting the one he loves back, and then being so unsure about how they actually feel. Remember that we see Ritsu's POV almost the entire series, but Takano isn't inside his head, he doesn't know what Ritsu is thinking. Ritsu sleeps with him and occasionally shows him some affection, but he isn't very reassuring. So even though Takano has that confident aura around him, he's actually in pain all the time, which can be seen in his intense and passionate gaze that just makes him pathetically and pitifully huggable. I love characters like that. Besides, Ritsu never would've taken initiative, so Takano had to do something to get their relationship started once more, and it's not like Ritsu was actually against it. He even says it himself that he could've pushed Takano away if he really didn't want any physical contact. Oh, also, Takano's he's really hot, but Nakamura-sensei's just not good at making characters look differently, because he looks a lot like Usami Akihiko from Junjou Romantica, but with black hair. With that being said, apart from the few resemblances and references to JR that I've made here, I wish people would stop comparing the two because ones for nothing to do with the other. I like SIH more, but it's got nothing to do with it.
Then there's Ritsu. He's adorable, and I love myself a tsundere, I just wish he'd make up his mind already. I can understand his inner conflicts, his sheltered life, his insecurities and his actual adult thoughts that homosexual relationships are hard...but he's far too in love to try to run away from it at this point. It's something he should've considered before, now it's too late and he should get it over with and confess.
My second favorite character is actually Yokozawa. He pissed me off at first, getting more and more annoying. Then the chapter that explained his point of view came out and I immediately understood him. How wouldn't someone fall in love with a person who is closed off to everyone else, but keeps only them so close? All of what he does is justified by his feelings and I can relate a hell of a lot to him. More than that, I feel bad for him. I'm so glad he got his own happy ending, I just wish it was longer or available in manga format. The little one-shots are nowhere near enough, so I'm happy we at least got the OVA, but I'd like much more.
I also love Isaka-san, always have and always will, since Junjou Romantica.
Even some of the side characters are well developed, like Ann-chan or Mino and his kid's relationship. Yes, they serve the purpose of impacting Takano and Ritsu's relationship, but it was thoughtfully constructed.
So now, the bad parts. The way their relationship has been developed is taking way too long, I think everyone agrees on that. Build up is great, but only if each interaction adds something new, another side to things, not just more of the same. For a manga about editors, Nakamura Shungiku needed a better editor herself. My initial impression was that it would have been better if Ritsu fell in love with Takano when they first meet again, and THEN found out who he was, because the way it actually happened made it clear that he already loved him, but refused to accept so himself, and the story dragged on from that...especially since it shows in the end of the chapter how long it will take for Ritsu to realize, which means we're expecting it. In any case, this seems to be a pattern with Nakamura-sensei's: the uke won't stop pretending like he doesn't want to, even when he does.
Also, I almost rolled my eyes at the "interfering" characters, and it's silly how the manga itself makes a reference to that without fixing its own issue. There's Haitani, Ann-chan, Nao... enough already.
My main problem with this manga, however, is probably controversial. I found myself skipping the chapters about the other two couples, and that's a huge part of the manga. They are cute as side characters, but neither relationship is as interesting as the main one, and I just thought of them as nuisances when their chapters came out instead of the main couple's or when I was reading and saw that the next chapter was about them. I did go back to read their stories once I finished the main couple's, but it wasn't as good. I think both of these relationships should be their own separate mangas, and then I could've read them with an adjusted expectation and not in the middle of Takano / Ritsu's drama. Both of the other relationships could've been one-chapter one-shots and that's that, there's nothing else to add to either of them, they aren't that interesting.
With all of that being said, Sekaiichi Hatsukoi is funny, cute, sexy, and gets my full attention until the end. Each volume makes me crave more - and believe me, that's a tough thing. Yes, SIH does take a little too long to get to the point and the chapters always follow the same format (a misunderstanding that generates a lot of angst, and then a resolution with sweet words and sex), but it's exactly what I look for in a series. A love that lasts for that long is too beautiful to even be put in words and it shows in every expression of Takano's how much he longed to see Ritsu again. It's just a series that makes me so happy to read, and for that I recommend it so much.
PS: I don't get why people complain about the art so much. Like I've expressed here, it shows the characters' feelings very well. Yes, the proportions may not be accurate, but this is just her personal style, haven't you guys ever read CLAMP?