It would depend on if you want the fortress to look dark and evil, or simple and light.
Dark and evil = Obsidian detail (and for strength in parts) and netherbrick for the large main areas.
Light and simple(ish) = Stone brick and cobble detail, maybe some blocks for decoration too if you can work them in. - patterns of glowstone, lapis or obsidian can be quite nice.
Tip: work in interesting designs/patterns with different (maybe unexpected) materials too, and use stairs or slabs to make odd shapes and angles
Allot will vary on the texture pack though, good luck and happy building!