Boku no Pico series is a good example of postive progression.
First episode - Boku no Pico - was preatty bad and apart from some music (I find the track that plays on the graveyard quite great) and joke potential it hadn't much going on for it. Pico himself isn't even cute, like at all. Watching it will get you hard, but you won't really want to fap, either because of it being too weird for you, or just not good enough.
Second episode - Pico to Chico - wasn't that great either, yet it was more watchable. Newly introduced character, titular Chico, is much cuter than Pico, yet he is even younger than him, so it might be more unpleasant to watch. Especially so, given that he doesn't exactly give consent to what Pico does to him. Also, some big boobs for a moment.
Third episode - Pico x Coco x Chico - feel to me like a major imporvement. The third main hero, Coco, is really convincing in his girlishness. I have even forgot for a moment, that he's a he. Anyway, he's surely the cutest one of the three, as far as girly cuteness is concerned, Chico is probably cuter in the boyish way (ignore that pic related is pretty much contrary to what I say). The story is actually somewhat (remember, this is hentai) engaging, with its slightly supernatural plot. And about what you were supposed to watch it for, this was the only episode which I fapped to, maybe it's because some time has passed between me watching first two and this one, I like to think it's just because it was better than them. Don't get me wrong though, it's great compared to the series itself.