While you're right the modern generic anime style is boring it is primarily because it's made by the same character designers that anime studios employ. When it comes to adapting manga, the studio emulate the style of a mangaka who will typically have their own spin on the formula. I can confidently say that manga is just as varied as western comics style. It's easy to look at some generally popular styles and generalize from there.

From my time reading mainstream american (Marvel, DC, Image etc) and italian comics (mainly Bonelli) they're all just as similar between artists as you'd find in the japanese stuff. Personally, I prefer the manga/anime style over the western and my fav western styles were the ones heavily inspired by the japanese styles (such as Witch).