If you watch or read Ecchi content, you have most likely acknowledged with Highschool DxD. It is the series that started the Ecchi era in the 2010s anime, in my opinion.

I won't be rating individual categories like Story, Art, Character, Villains. Instead, I am going to talk about every aspect and tell you my impressions. Without further ado let's get into the review.

Small warning: I'll try to write this review without any major spoilers, but I can't guarantee it.

Let's begin with the Story:
Issei Hyodo is killed by a fallen angel (no spoilers, he said) and made into a devil, so he decides to be a harem king. To accomplish it, he must become a high-ranking devil. Through his time with his new friends, he fights and defeats many enemies and learns about his comrades' past. That's the basic premise of Highschool DxD. Now, I'll begin with the aspects I like.
1, I like character development. I think that it has been done very well, considering it's an Ecchi. I'll probe into this a bit more in "characters"
2, I think story pacing is pretty good, making it easy to read. I remember I was bored of waiting for a new DxD episode to come out, so I started reading. I loved it. It was easy to read. I did finish reading one DxD volume every three days.
3, I like side stories. They give more depth to our main characters or even side characters. One good side story, in particular, is about Issei helping a character named Rizer Phoenix. I won't spoil anything, but it is one of the finest side stories with good story and comedy aspects.
4, I like Issei Ecchi's intercourse with his harem members, but that's also one big negative.
What I don't like:
1, Ecchi intercourses became way too repetitive and boring. Autor isn't allowed to write sex scenes into DxD, but I think it can be done in a 'civil' way. These intercourses are an issue with DxD and have become way too repetitive. Issei is constantly cockblocked by his harem members resulting in a cringy scene. You will like it at 1st, but it will start to bother you as you continue reading (or might not).

I'll take art very quickly:
I like the illustrations. I think they are pretty good. Some sexy stuff, some cool stuff, not much else to add. Maybe, I would like extra two or three illustrations, but that's a nitpick.

Now, Characters:
DxD has quite interesting characters, there are cliches, but overall it wasn't bothering me much. I liked most of the characters. Some of the characters are artificial Aholes (Written to repulse you, they are meant to be hated), but it was minor, and it won't play much of the role. What I like:
1, Interesting characters, there is a variety of personalities within DxD, coming from timid, kind personas to arrogant, cruel characters. I mean, every girl from Issei's harem has some unique trait, persona and he has quite a lot of girls in volume 25 and later Shin DxD.
2, Quite a good character development. When it comes to ecchi, a character's development sometimes feels either bland or just cheap (give a boring person a tragic backstory to make us feel sorry for him/her). I think DxD character development has done well, there are some things I dislike, but overall I think Ishibumi (author) did a good job.
3, There are few characters with a tragic backstory, but it isn't overdone, much to my luck. I hate when a show or book is trying to make someone we don't care about or hate a likable character due to 'tragic' backstory, DxD has some tragic backstories, but it's not overdone.
What I dislike:
1, The fact author isn't allowed to do sex scenes is screwing character development in the relationship and intimacy aspect. Sadly it is not his call, but it is the publisher's call, so we can't blame Ishibumi for this. It does hurt DxD a lot. Even though I don't desire DxD to become hentai, I would appreciate some intimacy advancement as DxD becomes a stalemate on that level. It goes into the Issei cockblocking cycles. That is probably the biggest flaw of DxD.

And now the weakest link in DxD, Villains:
Villains are the major setback to DxD. To put it simply: Villain appears, does some evil things, fights MC, Villain is on top, MC finds a miraculous way to defeat villain, MC defeats the villain. Something like this repeats a lot in DxD. The villains get boring quite quickly. After meeting one super strong character, soon to be replaced by another villain. I became bored with the villains. 1st villain was a good introduction, no masterpiece. She was decent, but she should have lasted 3-7 volumes or more. DxD villains follow quantity, but I think the quality would fit DxD way better.

Highschool DxD earns 8/10. If DxD had gone the quality villain route and had some sex scenes (it doesn't have to be full on Brazzers hardcore stuff) or something else than 100 same Ecchi intercourses, I would have given 9. If you like an Ecchi with an actual story, I can recommend you DxD.