This topic will show which of the various daedric artifacts and unique items that are improvable through smithing, and whether or not the items can be fully improved through any of the smithing perks.

Let me know if I missed anything, and I'll edit it in

Reading the lists:
Name of item......Improvable at all? - Tied to any perk? (which)

Weapons:

Blade of Woe...........Yes - Yes (steel)Blades Sword...........Yes - Yes (steel)Chillrend..............Yes - Yes (glass)Dawnbreaker............Yes - NoDragonbane.............Yes - Yes (steel)Ebony Blade............No - NoGauldur Bladeblade.....Yes - Yes (dwarven)Gauldur Blackbow.......Yes - Yes (dwarven)Ghostblade.............Yes - NoFiriniels End..........Yes - Yes (elven)Keening................No - NoMace of Molag Bal......Yes - NoMehrunes Razor.........Yes - NoNightingale Blade......Yes* - NoNightingale Bow........Yes - NoThe Longhammer.........Yes - Yes (orcish)The Pale Blade.........Yes - Yes (steel)The Rueful Axe.........Yes - NoValdrs Lucky Dagger....Yes - Yes (steel)Volendrung.............Yes - NoWindshear..............Yes - Yes (steel)Wuuthrad...............No - No* - the highest leveled version of the nightingale blade cannot be improved at all.

Armor - As there's tons of armor, consider anything unique NOT listed here to be Yes - No

Ancient Nord Armor..........Yes - Yes (daedric)Dragon Priest Masks (all)...Yes - Yes (daedric)Ebony Mail..................Yes - Yes (ebony)Falmer Armor................Yes - Yes (steel)Helm of Yngol...............Yes - Yes (advanced)Masque of Clavicus Vile.....No - NoSaviors Hide................Yes - Yes (ebony)Shield of Ysgramor..........No - NoShield of Solitude..........No - NoSpellbreaker................Yes - NoTarge of the Blooded........Yes - Yes (steel)Tumblerbane Gloves..........No - NoWolf Armor..................Yes - Yes (steel)

Should anyone from uesp.net or elderscrolls.wikia.com read this, feel free to port this info to the wikis.

Don't know if you're still looking for comments/suggestions or not, but if so ...

Savior's Hide can be tempered (leather) and is perked by ebony (weird, I know).

Added, thanks!

I didn't see the Ghostblade. It is improved by smithing with 3 ectoplasms if I remember correctly.

All pieces of the Nightingale set (armor, boots, gloves, hood) can be improved via Smithing with Void Salts.

I know you can improve the Pale Blade, I think it's tied to the Steel perk
Also the Gauldur Blackbow and Gauldur Blackblade can be improved and are tied to the Dwarven perk

Every Dragon Priest Mask requires a different perk to improve.

Not in my experience, and it's not what the http://uesp.net/wiki/Skyrim:Dragon_Priest_Mask#Dragon_Priest_Masks says either.
They do require different materials, but all of them are tied to daedric smithing to be improved fully.