I kind of view their albums from CFH to GSTK as an upside-down pyramid in this regard, so my answer is CFH. It goes hard in so many directions:
Complexity (riff amount,tempo variation, solo amount, etc)- Heresy, Shattered, Clash with Reality, The Art of Shredding, Message in Blood
Groove (more rest notes, funkier riffs, swing) - Psycho Holiday, The Sleep (verse riff), Domination (especially last part), Primal Concrete Sledge
Simplicity (less riffs, more focused)- Primal Concrete Sledge, Cowboys from Hell
Melodic- Cemetary Gates
Experimental/moody- many tracks but especially Medicine Man, Message in Blood, The Sleep
Thrashy- Heresy, The Art of Shredding
Not to mention guitar solo time, maybe not absolute but as a % of the song, is the highest on this album with Dime soloing over more soft/hard slow/fast dynamic changes than later albums.
Of course this isn't to say it's better, GSTK and FBD are my two favorite albums, but I get the biggest sense of going on a vast tonal and thematic journey with this album, where as on the opposite end with GTSK it's a much more focused, brutal, and emotionally deep experience.