Guide to Defensive (Tank) Items
Offensive Item Economy
The single biggest factor in what defensive items you make is what OFFENSIVE items you need. There are two main reasons why. Reason one: most comps rely on one or two units for damage, but more than that for frontline. For example on a 4 Trickshot 4 Bruiser board, your Kai'sa 2 is responsible for a much higher percent of your overall team damage than your Galio 2 is for frontline. Reason two: carry items are much more narrow than frontline items. A Kai'sa that has BiS without IE + LW (perhaps Shojin/Deathblade/GiantSlayer) is relatively much weaker than any 3 item frontliner without true BiS.
Thus, in this example, you should strongly avoid making Steadfast Heart in this comp since your Kai'sa wants two Gloves. Likewise, Vow and Crownguard should be built very sparingly in AP comps.
Bruisers
A common misconception is that since HP items give additional health on Bruisers, they are synergistic. This is incorrect due to OPPORTUNITY COST- since Bruisers already have high HP, adding resists and DR for multiplicative scaling is much more valuable. Additionally, in Set 11, many Bruisers (Sylas, Tahm Kench, Riven, Kobuko, Aatrox) have give themselves effective HP in the form of shields or healing with abilities that do not benefit from HP items, but do benefit from resists and DR.
Warmog's, Sterak's, and Steadfast Heart are less valuable on Bruisers.
Wardens
Percent damage reduction stacks multiplicatively. This can be very confusing since offensive percent damage bonus stacks additively. The easiest way to remember is that you almost always want to diversify your stats- in both of these cases, they stack unfavorably. You may still end up with a Redemption very often in 4 Warden + 4 Sniper due to Offensive Item Economy; what else will you do with excess Tears?
Redemption and Steadfast Heart are less valuable on Wardens.
Behemoths
Behemoths have high resists. As with Wardens and Bruisers, we want to diversify our stats, so resist items are weaker. Note that hero augments have some exceptions as Ethereal Blades benefits offensively from Bramble Vest, and Midnight Siphon benefits from Dragon's Claw max HP bonus and healing.
Stoneplate, Adaptive, Dragon's Claw, and Bramble are less valuable on Behemoths.
Vow and Crownguard
I classify Vow and Crownguard as pure tank items since they are very close, but they do trade off less defensives for mana and AP, respectively. Units that benefit from Vow's starting mana are Annie, Nautilus, Sett, and Udyr, who can CC their opposing counterpart first with Vow and sometimes kill them before they cast.
Units that benefit from Crownguard's AP are Sylas, Illaoi, Thresh, and Amumu- the AP benefits their entire ability, compared to something like Nautilus where it will do more damage but not affect the stun which is the main value.
Sunfire, Evenshroud, Spark
These are items that are mostly damage items but go on your frontline. Roughly 1/3rd of the value of these items are in the defensive stats while 2/3rd of the value is in offensive value. In the rare event you have an excess of frontline items, you can position an offtank with these behind a 3-item main tank. However, in 90%+ of games you should just put these with your other tank items- mainly because 2.33 items is better than 2 items, but also you want these items to survive too.
Titan's Resolve
This item gives zero defensive stats (besides the Chain used to build it) until the unit is hit ~25 times, at which point it is nearly dead. There are some exceptions such as Diana tank being shielded by Janna 3, but generally this item should not be built on traditional tanks.
Item Flexibility
With no open components, Chain Vest tends to be the safest carousel pick if you are looking for a pure tank item since it builds 5 of them (Vow, Crownguard, Bramble, Stoneplate, and Steadfast), and more importantly can combine with undesirable components such as Tear and Rod in AD comps. Cloak and Belt build less pure tank items, but can be preferable if you haven't secured antiheal and shred/sunder first.
Data Pitfalls
Steadfast Heart has the best delta out of any defensive item in 4 Wardens even though it is weaker than most alternatives. This is because Wardens are almost always played with Snipers, and having Steadfast Heart implies you have an excess of Gloves and thus already have some top tier items in the comp including Infinity Edge, Last Whisper, and Thief's Gloves. The stats for Steadfast Heart are also boosted by players surviving long enough to get this item in a Stage 5, Stage 6, or even Stage 7 anvil or carousel.
Data is very powerful, but it is not a replacement for your own critical thinking.
Video Version
If preferred, you can view a video version of this guide here: https://www.tft-coaching.com/videos/v/defensive-itemization!