They´re the most useful boots for a Tank, even if the enemy Team is full of AD champs, Tenacity can make the difference between Life and Death.
This Item is useful for every Team. It gives your teammates Armor, MR and a bit AD, buy it early in the game while laning.
Force of Nature is, in my opinion, THE must have for every Tank. It gives awesome Healthrege, MR and Movementspeed.
1370HP+45HP/5s.. No Comment
I know some are wondering about this but it´s quit nice to have AP for your Shields, an Aura for supporting your Mages and, last but not least the MR.

The sixth item? Feel free to choose everything you want, i like some more Armor like a "Thormail" against AD´s or a "Zhonya´s" for AP and against Karthus ulti (always nice to see him flaming about this^^), "Randiums Omen" or "Sunfire Cape" are also nice but if you know that you´re going to buy the Omen don´t forget the "Heart of Gold" in the Earlygame.