Well, if by 'best' we mean well - written and engaging, 'Re:Zero − Starting Life in Another World' is a great example. It has a very interesting plot that keeps you on the edge of your seat with its time - loop concept. Regarding 'mature', it does deal with some darker themes and complex emotions. But again, the 'Harme' is a bit of a mystery here. Maybe it was supposed to be 'harmony'? In that case, light novels that have a good harmony of different elements like 'No Game No Life' which combines strategy, comedy, and a unique world - building are great.
I'm not familiar with 'Harme' in this context. However, if we talk about mature light novels in general, 'Kumo desu ga, Nani ka?' (So I'm a Spider, So What?) can be considered. It has a unique perspective as the protagonist is reincarnated as a spider and has to survive in a harsh world. It deals with themes like survival and evolution which can be seen as more'mature' topics. It also has an interesting power - up system.
The combination of the angel's purity and the devil's allure in a love story is what makes it so appealing. It explores themes of forbidden love and defying expectations, which draws readers in and keeps them hooked.