Isekai portal fantasies offer a bridge between two types of fantasies, primary world fantasy and secondary world fantasy. Primary world fantasy, as described by J. R. R. Tolkien in "On Fairy-Stories", takes place on Earth, typically in the present at the time of writing. Examples of primary world fantasy include American Gods, The Dresden Files, and Who Fears the Devil?, with Soloman Kane, The Lord of the Rings, and arguably The Wheel of Time providing primary world fantasies in the past. Secondary world fantasy takes place in another world than Earth, such as Narnia, Westeros, Discworld, Lankhmar, or the scattered worlds of the Cosmere. Isekai takes main characters from the primary world and thrusts them into a secondary world adventure. Whether through a rabbit hole, a wardrobe, answering a strange personal advertisiment, or uploading one's consciousness into the internet, this transition between worlds is the defining feature of isekai. As this transition typically takes place in the first chapter, the story lives off the secondary world introduced to the reader.
While Western portal fantasies typically draw from sword-and-planet fantasy, myths, or fairy tales, Japanese light novels tend to draw from games for their conventions, with Dragon Quest being the primary influence--as discussed earlier on this site in "Blue Slime Fantasy." While isekai portals into actual MMO worlds are common, today's recommendations look at adventures in fantasy worlds unconstrained by silicon, even if the leveling and the looting remain.
Programmer Ichiro "Satou" Suzuki falls asleep in an overtime patching session he calls a "death march", only to wake up in a world that resembles the game he was working on. While the leveling and skill systems come straight from the game, he soon finds the world too real, and starts delving its secrets.
Sometimes a recommendation is on this list not because of quality, but because it is the purest example of the form. And since isekai stories are currently caught up in a search for novelty, twisting and riffing on the conventions of isekai, an example of what everyone is trying to subvert is required. Death March gets the nod over titles like In Another World with My Smartphone for navigating the traditional isekai conventions of ever-increasing cast, lands, powers, and quests while dropping the least characters and plot threads along the way. While Death March incorporates gaming tropes, it straddles the line between game world and fantasy world as other characters are Japanese souls reincarnated into the new world.
Death March also is notable as a "burnout" fantasy, where the main character is an overworked salaryman thrust into a new life, as opposed to the under-socialized teens that commonly fill light novels. The result is a more idyllic journey through the video game-inspired fantasy world, as Satou grows to enjoy the moment instead of just being married to work. Satou's age and maturity, compared to most isekai protagonists, filter out a number of pandering tropes as well.
When perennial loser and MMO junkie Kazuma Satou dies trying to save a girl from a runaway tractor, he finds himself in the waiting room of heaven, where, after a goddess roasts him for being an idiot, she gives him a choice. Kazuma can enter heaven, or take a continue in an MMO-inspired world as an adventurer. Kazuma naturally chooses the second option, complete with the customary choice of a starting cheat in the form of a legendary item or skill. Wanting to wipe the smug smirk from the goddess’s face, Kazuma selects her as his special perk. After all, what could be more powerful in a fantasy world than a goddess? To her horror, heaven agrees to his request and sends them both to the fantasy world. Now Kazuma and the goddess Aqua must quest to defeat the Demon King before either can return home.
A light-hearted comedy, Konosuba follows the other tradition of isekai light novels, that of flipping over one or more conventions. Here, the wish-fulfillment seen in many light novels gets turned on its head, as Kazuma's crusade against the Demon King is quickly laid low by misfortune and misfits, with none more dysfunctional than the goddess at his side, Aqua. The comedy is situational instead of gag-based, fueled by subverted expectations and a rare willingness to let the characters indulge in their faults--including the women. But no matter how genre-savvy Kazuma may act, he never treats his new world as just a game. Although my first review was rough on the series, later volumes do become more enjoyable, another trait common with many light novels.