Lugburz is a strong release, but less innovative than their later material. Plus, it really sits outside the basic 'Summoning' context, and, as such, is too unrepresentative to really be an appropriate answer to this question.
Minas Morgul is brilliant, but at times unsteady in its execution (as befits a band just working out the edges of a new style), and the sound is a bit - off.
Stronghold and Let Mortal Heroes Sing Your Fame both suffer in comparison to the bands other work. While both are excellent in the abstract, at times, they veer toward the formulaic and rockishly broad gestures rathern than the more subtle manipulations of texture and technique featured on other albums.
Oath Bound is internally more complex, and a worthy successor to Summoning's best material, but it is perhaps too narrow in its focus and feel - it simply lacks the diversity to appeal for every day listening.
The real pinnacle of the band's career to date were the recording sessions that produced both Dol Guldur (my choice, btw) and Nightshade Forests, and the material on these albums benefits from both the creative passion of Minas Morgul and the confident and assured execution of Oath Bound, with the added bonus of much greater variety in emotional and textural presentation. Neither album will ever get old.