What item to get over sunfire?

Sunfire is still good in a full build as it does give a lot of tankiness but other substitutes are things like Randuin's Omen for armor or Spirit Visage/Banshee's Veil for magic resist. If you're looking to replace Sunfire Cape in a full build then you would likely end up going with other types of tanky items such as the ones listed or a Guardian Angel but Sunfire Cape is very good in a full build and there usually isn't a need to substitute it.