the hardest one to do has been Sabbat. i am not trying to get everything released, but i do have a complete 7" collection. including some rare 7"s that were secretly released and not mentioned on discographys. i have several test pressings and jacket variations too.
i even made a special box that i keep them all in. i'll post a pic when i go back to my apartment.
some other completes i have are: Ghoul (jp), X/X Japan, Sacrifice (jp), Ground Zero (thought i had this a long time ago but recently found out about a demotape, and got it), Rosenfeld, Dementia, Shell Shock, Crowley, Casbah, Sabbrabells, Rommel, Balzac, Genoa, and Gism.
for Gism i even collect the test pressings and unofficial releases and have absolutly everything with all known variations. the hardest thing to find was a bootleg called Death Exclamations limited to like 200 copys with a super hardcore packaging/tons of inserts.