It's a tough choice. I don't remember what all the booklets of the CDs I own look like. There are some I do remember liking though, such as:

Janus - Vater (among other things, lyrics written in what resembles an old Bible, in biblical format)
Atrocity - Atlantis (lyrics written in code, making it really strenuous to read)
Eisregen - Farbenfinsternis (very graphic and well drawn pictures making it very clear why this album was "indiziert" in Germany)
Haggard - Awaking the Centuries (like Vater, lyrics written on really old paper)
Janus - Kleine Ängste (40 page booklet containing the story the lyrics were based on, with cool pictures)
Laibach - Opus Dei (I love how provocative and well done their, not so obvious, political satire is)