Plot - 3 Let me just start by saying the entire premise of the story is a lie. The title "The Greatest Demon Lord yada yada Average". One would assume from the title that the MC, a reincarnated demon lord, would be average, but nope. Magic has conveniently taken about 4 million steps backward since his death, making him conveniently overpowered. And that's not even the best part, He can unseal his lost powers as a demon lord when he wants to, which makes one question what the point of the story even is. Moving on to the actual plot, (or lack thereof) we see theMC beat up a certain cult that is trying to bring back "Evil Gods". Of course, what really happens here is the beating up of thugs that seem to have some vague and not-so-hidden goal. In 5-6 volumes we see next to no actual plot, just MC beating some old enemies from his era as a demon lord, and some new enemies that are all obviously associated with said cult. I will admit, however, that the fight sequences are fairly well written and decently immersive. That said, aside from the thoroughly uninteresting plot, the narrative and story-telling aren't anything special either.

Art - 6
Your generic LN art. The same as you would find any new-ish LNs.

Characters - 2
First off, we have our generic big-boobed generic fan service harem with a busty and pervy succubus and an innocent childhood friend. Both girls are plot devices that the MC sees as irreplaceable people to him (he was lonely in his past life etc etc) so he would go to any lengths to protect them. As such, MCs enemies obviously cannot keep their mouths shut about how they think they are burdens for him to carry. To talk about the girls themselves, they are pretty annoying and very cliche. Ginny, the succubus girl is just a device the author uses to give the childhood friend meaningless character development. Both girls are determined not to lose to each other to win the MCs heart, which spells more cliche and "I can do it!" type conversations.
I will admit that there were moments when I thought to myself "The MC isn't written too bad, is he?" only to have to show those words back where they came from when he behaves like a soppy shounen protagonist who cannot shut up about "nakama" and "friendship" and "people precious to him"

World Building - 7.5
This is pretty much the one good part about the LN. The MC talks about the past a lot, and there is also a volume where they actually get "immersed" in the past in a sense, so that helps a little bit.

Enjoyment - 3
Decent art with pretty girls and decent fight sequences. Pretty much it.

Overall - 4
Steer clear, not worth it. There are an absolute crap ton of fantasy LNs that are a million times better than this and are more worth your time.

To conclude, this is your generic fantasy lightnovel with an overpowered MC and a fan-service harem with shit plot and poor characters. Would not recommend.