You start with Doran's Shield, 1 health potion and warding totem
Against every single champion in top lane that deals mainly magical damage, you rush Spirit Visage
If you are against a champion that deals mainly physical damage, rush either sunfire cape or ninja tabi if the enemy deals mostly damage from physical attacks
Spirit of the Ancient Golem doesn't give you any resistances. It gives you 350 health, 10% CDR, 14 health regen, 35% cc reduction, the increased damage versus monsters and the extra bounty. You could rush this in matchups against champions like Irelia or champions that have only a chance of killing you in ganks. The health and CC reduction will help when getting ganked because you will be next to immune to CC with your W turned on. You COULD try going this item first if you think you can farm the jungle pretty often or if the enemy has like jungle rammus or fiddlesticks with their 3 second taunt/fear
I must admit I am unsure against which champions you can rush this item versus. I most of the times build this item as my last item of the core (sunfire, golem spirit,spirit visage) because I feel like the resistances from sunfire and spirit visage are superior.
So now for your core items, in no special order.
Against range ad like nidalee or jayce, or you just want health and health regen, go for 1-2 Doran's Shields
If the other team hasn't already surrendered, you can consider buying any of the items below
If game ain't over:
For gibbing people faster and poking
For Magic resist
You don't need much more magic resistance than spirit visage and from your runes and masteries. But chances are someone in your team hasn't built a locket of the iron solari already but he should have, so you can do it. The team with a Locket usually wins if the other team doesn't have one. There aren't many available choices for magic resist on tanks these days, health is more of a solution and you sure have it.
Reasoning behind not going warmogs: With the 3 core items: sunfire, spirit visage and golem spirit, you already have ~3k health and around 150 armor with tabis and 110 mr. The way Health and resistances work in this game is that for every 1k health, 75 of each resistances make the best defense so at 3k health the best way to compensate is to have 200 armor and 200 mr, but it can't usually be done. Warmogs would just be a bit over the top as you also gain health from Liandry's and a Randuins. With Liandrys and a Randuins + elixir of fortitude, you will have +4k health.
So after building sunfire, spirit visage and spirit of the ancient golem, you should consider which items are best next. If you need more magic resistance you should most of the times buy Banshee's Veil, but if you need armor on the same time you should get Guardian Angel.
Hidden trick with Guardian Angel: If you pop your ultimate right before the GA is popped, you regenerate health while respawning for 4 or 5 seconds.
If you want to get more damage you should get either a Thornmail or Liandry's. Thornmail if the enemy ADC is fed or they have a lot of physical damage dealers, and Liandry's if you don't need more survivability. Liandry's gives the most damage output.
So let's say you're up against a full ad team and maybe an ap support. Then you don't really need to build magic resistance and instead you can go for Ninja Tabi, Sunfire, Thornmail, Randuin's, Spirit visage and Frozen Heart. Spirit visage + frozen heart allows you to get 40% CDR