For me, 'That Time I Got Reincarnated as a Slime' stands out. The world-building and plot development are really engaging. Also, the characters have a lot of depth.
Well, it's a bit hard to pinpoint exactly which was the very first one. But some early examples could include 'El-Hazard: The Magnificent World', which had elements of the isekai genre.
The 'isekai walking manga' is pretty popular. It has managed to capture the attention of both seasoned manga readers and newcomers. The engaging storyline and vivid illustrations have made it a favorite among many.
Hentai manga isekai typically features outlandish settings and sexual content. The storylines can be quite unconventional and focus on fulfilling certain fantasies. However, it's important to note that such content may not be suitable for all audiences.
Isekai Quartet indeed has a manga. It offers a different perspective and sometimes delves deeper into the story compared to the anime. The manga can be a great companion to the animated version for fans wanting more of the Isekai Quartet world.
Another aspect is the focus on character growth. The protagonists usually start from being weak or inexperienced in monster breeding but gradually learn and develop their skills. They might gain new abilities or knowledge about different monster species through their adventures in the isekai world. In addition, these novels often have detailed descriptions of the monsters' appearances, behaviors, and abilities, which makes the world - building more vivid.
Adventure is also a big theme. Since it's an isekai setting, there are often new lands to explore. In these novels, the character might go on quests to find rare monsters to breed or special items that can enhance their breeding abilities. It could be traveling through dangerous forests or ancient ruins. The exploration aspect adds a sense of excitement and discovery to the story.
I'd say 'That Time I Got Reincarnated as a Slime' is up there. The story has a unique world-building and an engaging main character. Also, the art style is quite appealing.