+1 just because you mentioned that his passive isn't worth building around
Personally however I usually get Thornmail cause that combined with his ultimate makes some fun results
However you are better getting full Defence and using utility to get more mana or get the spell penetration if you don't like that

Hardened skin does not affect his ultimate damage at all. The reason I opted for Randuin's is that I wanted one big armor item and it seemed like the best choice since it also gives me some hp regen and cooldown reduction and has a nice active ability.

You are correct though. It does reduce the damage your ulti does, however, it caps at 80% bonus damage, that's eight hits. If you are using your ulti in group fights, like you should, it'll be capped anyways, and the slow will help you take less damage.

Getting Randuins and the Hardened Skin Mastery is a fail on Galio because it reduces his ult's damage. You want them to attack as they already are and not slow it up !?! And you don't want to block there Damage. Get Randuins out of there in my opinion , and switch it for Force of Nature or better yet Guardian Angel.