I agree with some of the previous posters that Summoning represents consistency in Metal more so than most bands - in my mind, more than any other band. Most of the best Metal bands have a lifecycle whereby which they release a solid debut album, an even stronger and more conceptually rich sophemore album, and usually another solid, though less conceptually consistent, third album before burning out of energy and either breaking up or regressing to a pattern of sub-par releases. Deicide, Morbid Angel, and Slayer, though they have realised more than three quality albums, all fall in line with this pattern. Is this pattern a reflection of the correlation between youthful energy of Metal and its inspiration? I would say so.
I think that the most consistent bands are usually the ones that are the most conceptually and ideologically firmly rooted. Summoning, Burzum, Graveland, and Averse Sefira all come to mind. This trait is more visible in Black Metal, which as I have noted in a previous post is more charachterized by a resurgance of belief and ideas, rather than the general lack of belief that charachterizes Death Metal.