Yes, I'm going to go there. I've actually been asked this more by my Japanese friend/supplier than anyone else, but I've always refrained from answering except to name one or two that I liked. However, I made the mistake of watching a few imouto anime on a binge a few weeks ago, and I began seriously considering which little sister paths were the best in the VNs I've played up until now. Note that I say these are the best 'paths', not necessarily the best imouto heroines. To be blunt, imouto heroines are a dime a dozen, and I've seen every variation on the concept a dozen times over, so I'm focusing on which paths were best, starting from ten, counting down to one.
First, little sisters in these will not include cousin paths unless they were raised under the same roof as siblings. Adopted sisters will be included, as will blood-related sisters who were parted from the protagonist for long periods of time.
10. Kuguyama Konoka (Prism Recollection by Clochette)
Like many of the best imoutos out there, Konoka partially stands out because of her attachment to her brother... which is often shown in odd ways (such as making perverted conversations with him and generally ignoring common sense if she doesn't like it). Konoka suffers from severe problems with both short-term and long-term memory, to the point where she is heavily reliant on diaries, pictures, and sketches to keep herself from losing everything she loves. Honestly, when I had to pick a Clochette imouto (and for an imouto list, there has to be one, since Clochette imoutos are usually so good), I picked her without any real hesitation.
9. Shimazu Akira (Komorebi no Nostalgica)
Akira is the hacker sister of the more 'normal' (though still capable in general) protagonist of the game. She is a genius, with a tendency toward relying on her brother and going crying to him when she messes up. Like all the paths of this game, her path ends up wrapped up in dealing with the issues of their times and dealing with the troubles surrounding Cinema's discovery.
8. Minori Hikari (Unmei Yohou o Oshirase Shimasu)
You may be wondering why I have never really pushed this VN despite the fact that this path is on my list... but there are good reasons. Hikari was manipulated into extreme dependence on her big brother from a very young age... and oniichan forgot that he did it. Generally speaking, the protagonist in this game is an awful person with horrible tendencies. However, this path was deliciously twisted and full of mutual dependence, so I had to put it here.
7. Kurosaki Sayo (Aoi Tori)
The relationship between Sayo and the protagonist of Aoi Tori is twincest at its best and worst. There is love, dependence, and all the twisted up relationship hangups you see in this kind of situation, coupled with the dark fate that hangs over their heads. Honestly, this path was both nicely twisted and full of the kind of passionate love you want to see in this kind of case, so I can definitely recommend it to incest lovers. It is, however, coupled with a bittersweet ending.
6. Hasuno Saki (Hapymaher)
Setting aside the fact that Saki is a 'fake' imouto, she nonetheless lived the life of Tohru's little sister for well over a decade for his sake, setting aside her romantic feelings and desires for the sake of helping him retain his sanity. This is not a small thing, as Saki's feelings toward him are blatantly obvious in her reactions when she discovers he has begun dreaming about Maia. I moved this one above Aoi Tori because Saki's relationship with Tohru is as deep as Sayo's with her brother, without the blood relationship to accelerate things.
5. Kaede (Shuffle Essence)
Say what you want about the translated version, but Kaede is one of the single most recognizable yandere imoutos in VN and anime history. Kaede's jealous reactions became a template for many later anime and games, and to this day, when you want a heroine's jealousy to come out of dependence, Kaede is the model. If I didn't have her on this list, someone would be pissed.
4. Tokitani Shinobu (Soshite Hatsukoi ga Imouto ni Naru)
To be honest, this relationship is one of the most heart-wrenching on this list, not the least because both the protagonist and Shinobu had no choice as children but to run away from their respective parents, out of fear for their lives and health. The way the protagonist 'adopted' Shinobu and devoted himself absolutely to her safety and protecting her from the harsh choices of their time as homeless runaways is one of the single most touching backstories I've seen in a VN. That this lifestyle didn't last long before they were taken in by old man Tatara doesn't erase the fact that they are ferociously devoted to one another, almost to the point of madness. Shinobu is quite obviously in love with the protag from the beginning, and I honestly never considered anyone else in the game for this role, despite the fact that the 'true' heroine is the other imouto character.
3. Saisu Riku (Floral Flowlove)
Riku, by design, is almost a non-person, an adviser in all paths but her own. However, in her path, the true path, her true nature becomes obvious... and it is a good one. I refuse to spoil things for you, and while the game itself is not unpredictable, the presentation is top class. On the surface she seems like your typical immature and selfish imouto character... but she is entirely something else once you get a chance to know her.
2. Asagiri Futaba and Ichiha (Realive)
Realive might not be Purple Software's best work (I still think Hapymaher is, though the Aoi Tori fanboys will argue with me over it), but the true path of Realive is rather obviously one of the best imouto paths ever. The twisting, intertwined relationship between the brother and his two sisters is so weirdly messed up that by the end of this VN you'll have trouble keeping straight just what happened. It is also an emotional rollercoaster that reminds me of many of the best nakige moments I've had over the past decade.
1. Shimazu Aki (Reminiscence)
Despite putting this in a spoiler box, I'm not going to spoil her path for you. Aki and her brother have an intensely antagonistic relationship that is full of actively lethal hatred and contempt. Aki seems to be living solely to see her brother suffer, and she only reluctantly participates in activities that might somehow help him. Of course, once you know the full story of what happened in their past and her brother's thoughts, you'll not only understand why but end up crying like a baby. The only downside of playing this path is that after you play it, you'll feel like playing the other paths is a betrayal and a form of blasphemy, lol.