The only ones I ever used specifically for HC was “F Hardcore” and “Hardcore Condolences” because I don’t want to manually reply after each death.
Are there any other HC addons that are must-have?
there used to be an addon called classicbestiary that told you all the abilities mobs have. this is essential because some mobs have “thrash” and this ability can deal a ton of damage all at once if you’re not careful 
the addon hasn’t been updated though and it no longer works. but maybe some other addon can do the same thing. you can also just memorize the list of mobs with thrash, that’s the only ability that matters.