Having spent the last couple years consuming almost every fanservice anime released by Madman Entertainment, I think I can safely sub-categorise anime fanservice into three categories:

1. Incidental. In the same way that essentially any movie or television show will find a way to exploit the attractiveness of their cast at some point – a shirtless scene here, a low-cut top there – almost every anime will at some point offer up some fanservice (more often that not, of their female characters). Even prestigious, idiosyncratic series like Neon Genesis Evangelion had their flutters of fanservice; it goes with the territory.
2. Obligatory. Much like it was difficult to tune into shows like Rome, Spartacus or Game of Thrones and not encounter a pair of unclothed boobs midway into the episode, these are the series with such consistent fanservice you can set your watch by them. These series tend to have overarching plotlines that, as the finale approaches, might even eclipse the importance of animated nudity, but you can expect some dubiously-justified bath scenes or panty shots pretty much every episode.
3. Soft Pornography. Pretty much what you’d expect. These shows might feint at storylines or character development, but boobs and ass are the priority, and you can expect well-established character traits – and the laws of physics – to be abandoned in an instant in the interest of tearing off a busty lady’s swimsuit. Battle scene storyboarding centres on torn blouses rather than action choreography.

Of course, as with any attempt to sub-categorise, there are limitations within these strata. For instance, I’d argue that most of High School DXD’s first two seasons are pretty well soft porn, but it’s so amusingly self-aware that it’s incredibly entertaining beyond its masturbatory motives. Meanwhile, that show’s third season was disappointing largely because it shifted into “Obligatory” mode, focusing its energies on a serialised plotline that wasn’t worth the attention.

Anyway, this all brings me to the fourth season of To Love-Ru, titled – imaginatively – Darkness 2nd following third season, Darkness. The show has undergone an interesting evolution over its many episodes, with its forgettable first season oscillating on the tamer end of the Obligatory category, spending a great deal of time on sci-fi and romance subplots that, again, didn’t warrant the time. Its second season amped up the fanservice significantly, but still tried to maintain the integrity of its continuing narrative – to its detriment.

Season three was a marked improvement by shifting towards soft porn. Oh, certainly, the quality of the show or its writing didn’t markedly increase, but by shifting attention from its lily-liver protagonist Rito, and Lala, his wacky inventor fiancé (of sorts) to said fiancé’s sister’s scheme to establish a harem, it found the right tone to suit its abundance of implausibly sexual entanglements. (Any time Rito finds himself in the presence of a female, it seems to end up with the lady in question divested of her clothes and/or brought into carnal closeness with Rito’s appendages.)

Season four, though, goes all out. Yes, there’s still a plotline of sorts – centring on the titular Golden “Darkness” (intergalactic assassin Yami) – but it’s a thin excuse for elaborate pandering to fanboy fetishes. Apparently aware that the only one still watching are sweaty-palmed otaku (and, uh, me), the writers amp up any fanservice instances to ridiculous extremes. When one of Lala’s inventions goes haywire – as they always do – it doesn’t merely vacuum up a bystander’s clothes, but an entire locker room full of teenage girls that somehow all topple atop a well-intended Rito. When the series’ Big Bad makes an appearance, her first task is to molest dozens of (briefly) swimsuit-clad with probing tentacles of animated water. Trying to assess the narrative quality of this sort of entertainment seems to be entirely beside the point.

So, that’s To Love-Ru Darkness 2 for you – an exemplar of the upper end of the Soft Pornography scale of fanservice anime. Any filthier than this, and you’d have to call it hentai.