I'll list sone bands where I think the debut album is their best album.
Sacramentum - Far away from the Sun
Demigod - Slumber of Sullen Eyes (duh, does anyone think this isn't their best full length?)
Dawn - Nær sólen gar niþer for evogher (All of Dawn's material is great, but this is their best imo.)
Dissection - The Somberlain (far better than SotLB)
Sarcofago - I.N.R.I
Ulver - Bergtatt
Agalloch - Pale Folklore (the only Agalloch I really bother with)
Mercyful Fate - Melissa (easily better than DBtO)
At the Gates - The Red in the Sky is Ours
Mayhem - De Mysteriis Dom Sathanas
Mortuary Drape - All the Witches Dance
Infernal Majesty - None Shall Defy
Septic Flesh - Mystic Places of Dawn
Nastrond - Toteslaut
Candlemass - Epicus Doomicus Metallicus
Emperor - In the Nightside Eclipse (I don't find anything past this debut to be worthwhile)
Enslaved - Vikingligr Veldi (shits on all their other full lengths that I've heard)
Darkthrone - A Blaze in the Northern Sky (not technically their debut, but their debut as a black metal band and a long time favourite of mine)